OGLE2-TR-L9b: An exoplanet transiting a fast-rotating F3 star

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1 Astrnmy & Astrphysics manuscript n. aa uvesplan c ESO 2008 December 3, 2008 OGLE2-TR-L9b: An explanet transiting a fast-rtating F3 star Snellen I.A.G. 1, Kppenhefer J. 2,3, van der Burg R.F.J. 1, Dreizler S. 4, Greiner J. 3, de Hn M.D.J. 1, Husser T.O. 5, Krühler T. 3,6, Saglia R.P. 3, Vuijsje F.N. 1 1 Leiden Observatry, Leiden University, Pstbus 9513, 2300 RA, Leiden, The Netherlands 2 Universitäts-Sternwarte München, Munich, Germany 3 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, D Garching, Germany 4 Institut für Astrphysik, Gerg-August-Universität Göttingen, Friedrich-Hund-Platz 1, Göttingen, Germany 5 Suth African Astrnmical Observatry, P.O. Bx 9, Observatry 7935, Suth Africa 6 Universe Cluster, Technische Universität München, Bltzmannstraße 2, D-85748, Garching, Germany ABSTRACT Cntext. Phtmetric bservatins fr the OGLE-II micrlens mnitring campaign have been taken in the perid All light curves f this campaign have recently been made public. Our analysis f these data has revealed 13 lw-amplitude transiting bjects amng stars in three Carina fields twards the galactic disk. One f these bjects, OGLE2-TR-L9 (P 2.5 days), turned ut t be an excellent transiting planet candidate. Aims. In this paper we reprt n ur investigatin f the true nature f OGLE2-TR-L9, by re-bserving the phtmetric transit with the aim t determine the transit parameters at high precisin, and by spectrscpic bservatins, t estimate the prperties f the hst star, and t determine the mass f the transiting bject thrugh radial velcity measurements. Methds. High precisin phtmetric bservatins have been btained in g, r, i, and z band simultaneusly, using the new GROND detectr, munted n the MPI/ESO 2.2m telescpe at La Silla. Eight epchs f high-dispersin spectrscpic bservatins were btained using the fiber-fed FLAMES/UVES Echelle spectrgraph, munted n ESO s Very Large Telescpe at Paranal. Results. The phtmetric transit, nw mre than 7 years after the last OGLE-II bservatins, was re-discvered nly 8 minutes frm its predicted time. The primary bject is a fast rtating F3 star, with vsini=39.33±0.38 km/s, T=6933±58 K, lg g = 4.25±0.01, and [Fe/H] = 0.05±0.20. The transiting bject is an extraslar planet with M p =4.5±1.5 M Jup and R p =1.61±0.04R Jup. Since this is the first planet fund t rbit a fast rtating star, the uncertainties in the radial velcity measurements and in the planetary mass are larger than fr mst ther planets discvered t date. The rejectin f pssible blend scenaris was based n a quantitative analysis f the multi-clr phtmetric data. A stellar blend scenari f an early F star with a faint eclipsing binary system is excluded, due t the cmbinatin f 1) the cnsistency between the spectrscpic parameters f the star and the mean density f the transited bject as determined frm the phtmetry, and 2) the excellent agreement between the transit signal as bserved at fur different wavelengths. Key wrds. stars: planetary systems - techniques: phtmetric - techniques: radial velcity 1. Intrductin Transiting extraslar planets allw direct measurements f their fundamental parameters, such as planet mass, radius, and mean density. Furthermre, their atmspheres can be prbed thrugh secndary eclipse bservatins (e.g. Charbnneau et al. 2005; Deming et al. 2005; Knutsn et al. 2007), and atmspheric transmissin spectrscpy (e.g. Charbnneau et al. 2002; Tinetti et al. 2007; Snellen et al. 2008). This makes transiting explanets f great scientific value. Many phtmetric mnitring surveys are currently underway. Several f these surveys are very successful, such as the transit campaigns f the Optical Gravitatinal Lens Experiment (OGLE-III; 7 planets; e.g. Udalski et al. 2008), the Trans- Send ffprint requests t: snellen@strw.leidenuniv.nl Atlantic Explanet Survey (TrES; 4 planets; e.g. Mandushev et al ), the Hungarian Autmated Telescpe Netwrk (HATNet; 9 planets; e.g. Shprer et al. 2008), the XO survey (5 planets; e.g. Burke et al. 2007), and the Wide Area Search fr Planets (SuperWASP; 15 planets; e.g. Andersn et al. 2008). In additin, CRT is targeting planet transits frm space (4 planets; e.g. Aigrain et al. 2008), ultimately aimed at finding Earth r super-earth size planets. In this paper we present a new transiting extraslar planet, OGLE2-TR-L9 b. The system has an I-band magnitude f I=13.97, and an rbital perid f 2.5 days. The hst star is the fastest rtating and httest (main sequence) star arund which an rbiting extraslar planet has been detected t date. The transit system was the prime planetary candidate frm a sample f thirteen bjects presented by Snellen et al. (2007;

2 2 Snellen et al.: An extraslar planet transiting a fast-rtating F3 star F g F i OGLE2 TR L HJD d Fig. 1. Transit lightcurves f OGLE-TR-L9 in g, r, i and z bserved simultaneusly with the GROND instrument, munted n the MPI/ESO 2.2m telescpe. The line shws the best mdel fit fr the cmbined light curves, as discussed in the text. S07). They were drawn frm the nline database f the secnd phase f the OGLE prject, a campaign primarily aimed at finding micrlensing events, cnducted between 1997 and 2000 (OGLE-II; Szymanski 2005; Udalski et al. 1997). The lw amplitude transits were discvered amng the light curves f stars, with 13.0<I<16.0, lcated in three Carina fields twards the galactic plane. Nte that the light curve data have a different cadence than usual fr transit surveys, with 1 2 phtmetric pints taken per night, ttalling epchs ver 4 years. This wrk shws that such a data set is indeed sensitive t lw-amplitude transits, and can yield transiting extraslar planets. In sectin 2 f this paper we present the analysis f new transit phtmetry f OGLE2-TR-L9, taken with the GROND instrument munted n the MPI/ESO 2.2m telescpe at La Silla. The particular gals f these bservatins are the rediscvery f the transit, imprved transit parameters and rbital ephemeris. In sectin 3 the spectrscpic bservatins with the FLAMES/UVES multi-fiber spectrgraph n ESO s Very Large Telescpe are described, including a descriptin f the analysis f the radial velcity variatins, f the bisectr span, and the spectrscpic parameters f the hst star. In sectin 4 and 5 the stellar and planetary parameters with their uncertainties are determined, and arguments against a stellar blend scenari laid ut. The results are discussed in sectin Transit phtmetry with GROND 2.1. Data acquisitin and analysis We bserved ne full transit f OGLE-TR-L9 with GROND (Greiner et al. 2008), which is a gamma ray burst fllw-up instrument munted n the MPI/ESO 2.2m telescpe at the La F r F z Table 1. Limb-darkening cefficients used fr the transit fitting, taken frm Claret (2004) fr a star with metallicity [Fe/H]=0.0, surface gravity lg g=4.5, and effective temperature T e f f =7000K. filter γ 1 γ 2 g r i z Silla bservatry. GROND is a 7-channel imager that allws t take 4 ptical (g r i z ) and 3 near infrared (JHK) expsures simultaneusly. On January 27, 2008, a ttal f 104 images in each ptical band and 1248 images in each near infrared band were taken. The JHK-images turned ut t have insufficient signal t nise t detect the transit, and will nt be cnsidered further. Using expsure times f 66 secnds and a cycle rate f 2.5 minutes, we cvered a perid f abut 4 hurs centered n the predicted transit time. All ptical images have been reduced with the mupipe sftware develped at the University Observatry in Munich 1. After the initial bias and flatfield crrectins, csmic rays and bad pixels were masked and the images resampled t a cmmn grid. The frames did nt suffer frm detectable fringing, even in z-band. Aperture phtmetry was perfrmed n OGLE2-TR-L9 and eight t ten interactively selected reference stars, after which light curves were created fr each f the 4 bands. The aperture radius was chsen t be 12 pixels, crrespnding t 1.9 arcsecnds, with a seeing f typically 1.1 arcsecnds during the bservatins. The sky was determined in an annulus between 20 and 30 pixels frm the bject psitins. The rms in the individual light curves f the reference stars was in all cases better than 0.3%. This resulted in typical precisins f the relative fluxes better than 0.2% Fitting the transit light curves The lightcurves in g, r, i, and z, were fitted with analytic mdels as given by Mandel & Agl (2002). We use quadratic limb-darkening cefficients taken frm Claret (2004), fr a star with metallicity [Fe/H]=0.0, surface gravity lg g=4.5, and effective temperature T eff =7000K (very clse t the spectrscpic parameters f the star as determined belw). The values f the limb-darkening cefficients are given in Table 1. Using a simultaneus fit t all 4 lightcurves we derived the mean stellar density, M star / R 3 star in slar units, the radius rati R planet/ R star, the impact parameterβ impact in units f R star, and the timing f the central transit. Tgether with a scaling factr fr each band, there were eight free parameters t fit. The light curves and the mdel fits are shwn in Fig. 1., and the resulting parameters are listed in Table 2. All lightcurves fit well t the mdel except fr the g-band lightcurve, which is attributed t the significantly mre nisy light curve, and the prly determined baseline, particularly befre ingress. 1 arri/mupipe/

3 Snellen et al.: An extraslar planet transiting a fast-rtating F3 star 3 Table 2. The transit, hst star, and planetary cmpanin parameters as determined frm ur phtmetric and spectrscpic bservatins. Fig. 2. The central part f ne rder f the cmbined UVES spectrum f OGLE2-TR-L9, with verpltted a synthetic spectrum with T=6900 K, g=4.5, [Fe/H]=0, and vsini=39.33 km/sec. 3. Spectrscpic Observatins with UVES/FLAMES We bserved OGLE2-TR-L9 with the UV-Visual Echelle Spectrgraph (UVES; Dekker et al. 2000), munted at the Nasmyth B fcus f UT2 f ESO s Very Large Telescpe (VLT) at Paranal, Chile. The aims f these bservatins were t estimate the spectrscpic parameters f the hst star, and t determine the radial velcity variatins. The bservatins were perfrmed in fiber mde, with UVES cnnected t the FLAMES fiber facility (Pasquini et al. 2002), with 7 science fibers and with simultaneus thrium-argn wavelength calibratin (UVES7 mde). Apart frm ur main target, fibers were allcated t tw ther OGLE-II transit candidates frm S07 (OGLE2-TR-L7 and OGLE2-TR-L12), and three randm stars within the 25 FLAMES field. In additin, ne fiber was psitined n empty sky. A setup with a central wavelength f 580 nm was used, resulting in a wavelength cverage f Å ver tw CCDs, at a reslving pwer f R= Since the upper CCD turned ut t cver nly a small number f strng stellar absrptin lines, in additin t suffering frm significant telluric cntaminatin, nly the lwer CCD ( Å) was used fr further analysis. Eight bservatins were taken in Directr s Discretinary Time, in service mde in the curse f December 2007 and January 2008, spread in such a way that the data wuld be evenly distributed in rbital phase f ur main target (see table 3). The data were analysed using the midas-based UVES/FLAMES pipeline as prvided by ESO, which results in fully reduced, wavelength calibrated spectra. Since we were cncerned abut the wavelength calibratin f the fifth epch (see belw), we als analysed the data using purpse-build IDL rutines. N significant differences in the wavelength slutins were fund. The resulting signal-t-nise per reslutin element, in the central part f the rders, varies between 10 and 20 ver the different epchs (see table 3). Transit: ρ s = ±0.0091ρ sun R p /R s = ± β impact = ± t 0 = ± HJD P = ± d Hst star: Crd. (J2000) = 11 h 07 m s I mag = I J mag = 0.466±0.032 J K mag = 0.391±0.049 T = 6933±58 K lg g = 4.47±0.13 lg g = 4.25±0.01 [Fe/H] = 0.05±0.20 vsini = 39.33±0.38 km/s R s = 1.53±0.04 R sun M s = 1.52±0.08 M sun Age < 0.66 Gyr Planetary Cmpanin: K = 510±170 m/s i = 79.8±0.3 a = ± AU R p = 1.61±0.04 R jup M p = 4.5±1.5 M jup Determined frm the spectrscpic analysis Determined frm mean stellar density cmbined with the evlutinary tracks Table 3. Spectrscpic bservatins f OGLE2-TR-L9 taken with UVES/FLAMES. The first three clumns give the Helicentric Julian Date, the planet s rbital phase at the time f bservatin, and the signal-t-nise rati f the spectra per reslutin element in the center f the middle rder. Clumn 4 and 5 give the radial velcity and the bisectr span measurements. HJD Orbital SNR RV BiS Phase km s 1 km s ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Determinatin f stellar spectrscpic parameters The spectrscpic parameters f the star, vsini, surface temperature, surface gravity, and metallicity, were determined frm the SNR-weighted, radial velcity shifted cmbinatin f the eight epchs taken with UVES. This cmbined spectrum has a signal-t-nise rati f 42 in the central areas f the rders. Detailed synthetic spectra were cmputed using the interactive data language (IDL) interface SYNPLOT (I. Hubeny, private cmmunicatin) t the spectrum synthesis prgram SYNSPEC

4 4 Snellen et al.: An extraslar planet transiting a fast-rtating F3 star (Hubeny et al. 1995), utilising Kurucz mdel atmspheres 2. These were least-squares fitted t each individual rder f the cmbined UVES spectrum. The final atmspheric parameters were taken as the average values acrss the available rders. The uncertainties in the fitted parameters estimated using a χ- square analysis and frm the scatter between the rders, give similar results, f which the latter are adpted. The best fitting parameters and their uncertainties are given in table 2. One rder f the cmbined UVES spectrum is shwn in figure 2, shwing the Mg b 5170Å cmplex, with the synthetic spectrum with T=6900 K, lg g=4.5, [Fe/H]=0, and vsini = km/sec, verpltted Radial velcity measurements The rders f the eight spectra were first csine-tapered t reduce edge effects. Crss-crrelatins were perfrmed using the best-fitted velcity-bradened synthetic spectrum, as determined abve, as a reference. The spectrum f the sky-fiber indicated that the sky cntributin was typically f the rder f 0.5%. Hwever, fr the bservatin at 0.74 rbital phase (epch 5), the relative sky levels were an rder f magnitude larger, due t a cmbinatin f bad seeing and full mn. We therefre subtracted the sky spectrum frm all target spectra befre crss-crrelatin. The resulting radial velcity data are listed in table 3, crrected t helicentric values. The uncertainties are estimated frm the variatin f the radial velcity fits between the different rders. The final radial velcity data as functin f rbital phase (the latter determined frm the transit phtmetry), are shwn in figure 3. The data were fitted with a sine functin with the amplitude f the radial velcity variatins, K, and a zerpint, V 0, as free parameters. The radial velcity amplitude was determined at K= 510±170 m/s, with V 0 =+0.2 km/s. We als determined the variatins f the bisectr span (fllwing Quelz et al. 2001) as functin f radial velcity and rbital phase. These are shwn in figure 4. We least-squares fitted the bisectr span measurements as functin f rbital phase with a sinusid, but n significant variatins at a level f 0.01±0.140 km s 1 are fund. Althugh this means that there is n indicatin that the measured radial velcity variatins are due t line shape variatins, caused by either stellar activity r blends f mre than ne star, the errrs are very large, making any claim based n the bisectr span rather uncertain. Fig. 3. The radial velcity measurements f OGLE2-TR-L9 as functin f rbital phase frm the ephemeris f the transit phtmetry. 4. Estimatin f the stellar and planetary parameters 4.1. Stellar mass, radius, and age The transit phtmetry prvides an estimate f the mean density f the hst star, while the spectrscpic bservatins yield its surface temperature, surface gravity, and metallicity. The stellar evlutinary tracks f Siess et al. (2000) were subsequently used t estimate the star s mass, radius, and age, resulting in M s =1.52±0.08 M sun, R s =1.53±0.04 R sun, and an age 2 Fig. 4. Bisectr variatins as functin f rbital phase (tp panel) and radial velcity value (bttm panel). The slid line and dashed lines in the tp panel indicate the least-squares fitted sinusidal variatin in the bisectr span and its uncertainty at 0.01±0.140 km s 1. f <0.66 Gyr. These parameters crrespnd t a surface gravity f lg g=4.25±0.01, which is in reasnable agreement, but abut 1.7σ lwer than the spectrscpic value. It shuld be realised that it is ntriusly difficult t btain reliable lg g values frm relatively lw signal t nise spectra.

5 4.2. Planetary Mass and Radius Using the values btained frm the transit fit t the GROND light curves, the radial velcity fit, and the stellar parameters as derived abve, we btain a planetary mass f M p = 4.5±1.5 M jup and a planetary radius f R p = 1.61±0.04 R jup. The semimajr axis f the rbit is at a=0.0308± AU. The mean density f the planet is 1.44±0.49 g cm 3. This means that OGLE2-TR-L9b is ne f the largest knwn transiting ht Jupiters, nly TrES-4b and WASP-12b are marginally larger, althugh its mean density is similar t that f Jupiter. Even s, OGLE2-TR-L9b is significantly larger than expected fr an irradiated 4.5 M Jup planet (Fressin et al. 2007). Snellen et al.: An extraslar planet transiting a fast-rtating F3 star 5 5. Rejectin f blended eclipsing binary scenaris Large phtmetric transit surveys are prne t prduce a significant fractin f false interlpers amng genuine transiting extraslar planets. If the light frm a shrt-perid eclipsing stellar binary is blended with that frm a third, brighter star, the cmbined phtmetric signal can mimic a transiting explanet. Althugh the radial velcity variatins induced by an eclipsing binary shuld be rders f magnitude larger than thse caused by a planet, the blending f the spectral lines with thse frm the brighter, third star culd prduce variatins in the veral crss-crrelatin prfile that have significantly smaller amplitudes, pssibly as small as expected fr giant planets. Since this wuld be accmpanied with significant line-shape variatins, bisectr span analyses are ften used t reject a blended eclipsing binary scenari. Althugh n significant variatins in the bisectr span are bserved in OGLE2-TR-L9, it culd be argued that this is due t the lack f signal-t-nise. We shw hwever that a blended eclipsing binary scenari can be rejected anyway, because f the fllwing bservatins: 1 Transit light curves frm g t z band: As can be seen in Fig. 1, there is an excellent agreement between the light curves frm g t z band. This means that if the transit was actually caused by a backgrund eclipsing binary blended with a bright fregrund star, the clrs (and thus the surface temperatures) f the eclipsed binary star and fregrund star shuld be very similar. 2 Transit shape and spectral classificatin: The mean stellar density as determined frm the transit phtmetry is in excellent agreement with the spectral classificatin, bth cnsistent with an early F star. Using the argument abve, this means that if this is a blend, then bth the fregrund star and the eclipsed binary star shuld be early F stars. Hwever, if we nw assume that a significant fractin f the light cmes frm a fregrund star, and we remve this cntributin frm the light curve, the transit can n lnger be fitted by an early F star, but nly by a star f significantly higher mean density, implying a cler, less massive star, which is again in cntradictin with pint 1). This means that the early F star is the transited bject, and that a blended eclipsing binary scenari can be rejected. Fig. 5. The upper panel shws the stellar surface temperature, T s, versus the mean density,ρ s, fr evlutinary tracks f Siess et al. (2000). The filled squares n each track indicate stellar ages f 0.1, 0.5, and years, (and years fr M 1.2M Sun ), with larger symbls indicated higher ages. The dashed line indicates the maximum pssibleρ s fr a given stellar surface temperature. The bttm panel shws the cnfidence intervals frm the χ-square analysis f all pssible blended eclipsing binary scenaris fitted t the GROND light curves, with n the x-axis the difference in surface temperature between the eclipsed and the third star, and n the y-axis the fractin f the ttal light cming frm the third star in r -band. It shws that the cmbined g r i z light curves can nly be fitted including a lw level ( 30%) f light cntaminatin. T further explre the pssible rle f additinal light frm a blended star, we perfrmed a quantitative analysis, simulating backgrund eclipsing binary systems with their light diluted by that frm a third star. We first used the stellar evlutinary tracks f Siess et al. (2000) t determine the full range f stellar parameters that can be present in eclipsing binaries, f which

6 6 Snellen et al.: An extraslar planet transiting a fast-rtating F3 star nly the stellar surface temperature, T ecl, and the mean stellar density,ρ ecl f the eclipsed star are f interest fr the simulatins. Nte that the evlutinary status f the third star is nt imprtant, since we d nt restrict urselves t physical triple systems, but als include chance-alignments f back- and fregrund stars. As is indicated in the tp panel f figure 5, where the stellar evlutinary tracks are shwn, there is a maximum pssible mean-stellar density fr a given surface temperature. This was used as a bundary cnditin in the simulatins. In ur simulatins we varied tw parameters, 1) the difference between the surface temperature f the eclipsed star and that f the third star, T ( 1000 K< T<+1000 K), and 2) the fractin f light cming frm the third (pssibly unrelated) star, F 3rd (0<F 3rd < 99%). The cmbined light f the eclipsing binary and third star shuld prduce a spectrum which is best fitted with a surface temperature f T cmb = 6933 K. Therefre a simple linear relatin between T cmb and the surface temperatures f the individual stars was assumed, such that T ecl = T cmb F 3rd 1+F 3rd T. Nte that any small fractin f light that culd be cming frm the eclipsing star is simply added t F 3rd. In this way, each cmbinatin f F 3rd and T, results in a T ecl and a maximum pssibleρ ecl. It als results in a fractinal cntributin f light frm the third star that varies ver the fur filters. Subsequently, fr each cmbinatin f F 3rd and T, mdel eclipsing binary light curves were least-squares fitted t the g r i z GROND data, using as befre the algrithms f Mandel & Agl (2002), in which the binary size rati and the impact parameter were cmpletely free t vary, andρ ecl was retricted t be belw the upper limit set by T ecl. In this way, all pssible blended eclipsing binary scenaris are simulated, independently f whether the third star is physically related t the binary r nt. The bttm panel f figure 5 shws the cnfidence cnturs f the χ-square analysis f all pssible blended eclipsing binary scenaris. It shws that the cmbined g r i z data can nly be fitted by light curves f eclipsing binaries with a lw level 30% (90% cnfidence level) f blended light, meaning that mst light in the stellar spectrum must cme frm the eclipsed star. Scenaris in which the stellar spectrum is dminated by a third star with a small cntributin frm a backgrund eclipsing binary, can be strngly rejected. The transit light curves prduced by thse rejected scenaris are simply t wide, and/r t V-shaped, and/r t clr dependent t fit the GROND data. One scenari that we cannt reject, is a small cntributin frm a blended star. Fr example, it culd in principle be pssible that the light frm the transited F3 star is diluted at a 30% level with light frm anther F star (with a similar vsini and radial velcity, therwise it wuld shw up in the spectra). In this case, the transiting planet wuld be 30% mre massive (and 15% larger) than determined abve, by n means mving it utside the planet mass range. Nte that fr mst transiting planets presented in the literature, such a lw-level cntaminatin scenari can nt be excluded, since the variatins in the bisectr span wuld be rders f magnitude smaller than in the case f a blended eclipsing stellar binary. This is because the radial velcity variatins in the latter case are times larger than in the first case. There have been several reprts f blended eclipsing binaries hiding ut as transiting planets, mst ntably by Mandushev et al. (2005), and Trres et al. (2004). Hwever, these studies dealt with very lw signal-t-nise light curves, and the true nature f these systems wuld have been easily brught t light by such high quality phtmetric data as presented in this paper. Mandushev et al. (2005) rejected a transiting planet scenari fr the fast rtating (vsini=34 km/s) F5 star GSC , in favur f a blended eclipsing binary. This system was shwn t cnsist f a hierarchical triple cmpsed f an eclipsing binary with G0V and M3V cmpnents, in rbit arund a slightly evlved F5 dwarf. The latter star in this scenari cntributes fr 89% t the ttal light frm the system. Althugh they claim that the true nature f this system was nt revealed by their BVI light curves, the clr difference between the G0V and F5V star means that the transit must be 25-30% deeper in I-band than in B-band. Hwever, n quantitative analysis f the light curves was presented, and the authrs claim that the true nature f the system was nly brught t light by spectrscpic means. In a similar fashin Trres et al. (2004) presented the case f OGLE-TR-33, which was identified as a triple system cnsisting f an eclipsing binary with F4 and K7-M0 cmpnents rbiting a slightly evlved F6 star. Hwever, their phtmetry relied slely n the riginal I-band OGLE-III data, resulting in a relatively lw SNR transit detectin with the ingress and bttm f the transit nt well cvered. They als claim that the blended eclipsing binary is nly revealed by spectrscpy. Hwever, their best fitting planet mdel already pinted twards a very unlikely planet radius f 3 R Jup, and the V-shaped transit prduced by the blended eclipsing binary wuld have been easily picked up by ur high precisin phtmetry. Nte that while Trres et al. (2004) and Mandushev et al. (2005) nly cnsider physical triple systems, ur analysis presented abve includes all pssible scenaris, als thse invlving chance-alignment f backgrund r fregrund stars. 6. Discussin Mre than seven years and >1000 rbital perids after the last bservatins f OGLE2-TR-L9, the transit signal was rediscvered nly 8 minutes frm its predicted time (frm S07). It nt nly shws that an bserving campaign with large time intervals between measurements can prduce reliable light curves, it als shws it prduces extremely accurate rbital perids. OGLE2-TR-L9b is the first extraslar planet discvered transiting a fast rtating (vsini=39 km/s) F star. OGLE2-TR-L9 is als the star with the highest surface temperature (T=6933 K) f all main sequence stars that hst an explanet knwn t date. It is therefre nt surprising that the uncertainties in the radial velcity variatins are higher than fr mst ther transiting explanets presented in the literature. Only due t the high mass f OGLE2-TR-L9b, we were able t detect its radial velcity signature. Nte hwever that, since a blend scenari can be rejected at high significance, an upper limit t the mass f OGLE2-TR-L9b wuld have been sufficient t claim the presence f a transiting extraslar planet, althugh with an unknwn mass. Similar arguments may have t be used in the

7 Snellen et al.: An extraslar planet transiting a fast-rtating F3 star 7 case f future detectin f transits f Earth-size planets frm Kepler r CRT, since their radial velcity signature may be t small t measure. OGLE2-TR-L9b has a significantly larger radius than expected fr a planet f abut 4.5 times the mass f Jupiter, even if it is assumed that 0.5% f the incming stellar luminsity is dissipated at the planet s center (Fressin et al. 2007). Hwever, it is nt the nly planet fund t be t large (e.g. CRT-ex- 2b, TrES-4b, and XO-3b). Several mechanisms have been prpsed t explain these blated radii, such as mre significant cre heating and/r rbital tidal heating (see Liu, Burrws & Ibgui 2008 fr a recent detailed discussin). The measured vsini and estimated stellar radius cmbine t a rtatin perid f the hst star f 1.97±0.04 days. It means that the rtatin f the star is nt lcked t the rbital perid f OGLE2-TR-L9b. A vsini f 39 km/sec is within the nrmal range fr stars f this spectral type. The mean vsini f F5 t F0 stars in the slar neighburhd range frm 10 2 t 10 3 km/sec respectively. Nte that the vsini f OGLE2-TR- L9a is nly 9% f the expected break-up velcity fr a star f this mass and radius. Assuming the general Rche mdel fr a rtating star (e.g. Seidv, 2004), the rati f plar t equatrial radius f OGLE2-TR-L9a will be n the rder f, (v/v max) Thus, the rtatinal flattening f the hst star is nt expected t significantly influence the transit shape. OGLE2-TR-L9 is expected t exhibit a strng Rssiter- McLaughlin effect. Simulatin using a segmented stellar surface predict an amplitude f 230 m/sec. Seidv Z., The generalized Rche mdel, 2004, astr-ph/ Shprer, A., et al. 2008, ApJ submitted, arxiv: Siess, L., Dufur, E., & Frestini, M. 2000, A&A, 358, 593 Snellen, I. A. G., van der Burg, R. F. J., de Hn, M. D. J., & Vuijsje, F. N. 2007, A&A, 476, 1357 (S07) Snellen, I. A. G., Albrecht, S., de Mij, E. J. W., & Le Ple, R. S. 2008, A&A, 487, 357 Szymanski, 2005, Acta Astrn., 55 Tinetti, G., et al. 2007, Nature, 448, 169 Trres G., Knacki M., Sasselv D., Saurabh J., 2004, ApJ 614, 979 Udalski, A., Kubiak, M., & Szymanski, M. 1997, Acta Astrnmica, 47, 319 Udalski, A., et al. 2008, A&A, 482, 299 Acknwledgements. We thank the annymus referee very much fr his r her insightful cmments. Based n bservatins cllected at the Eurpean Organisatin fr Astrnmical Research in the Suthern Hemisphere, Chile (280.C-5036(A)) T.K. acknwledges supprt by the DFG cluster f excellence Origin and Structure f the Universe. References Aigrain, S., et al. 2008, A&A Let. in press, arxiv: Andersn, D. R., et al. 2008, MNRAS, 387, L4 Burke, C. J., et al. 2008, ApJ submitted, arxiv: Charbnneau D., Brwn T.M., Nyes R.W., Gilligand R.L., 2002, ApJ 568, 377 Charbnneau D., Allen L., Megeath S., Trres G., Alns R., Brwn T., Gilliland R., Latham D., Mandushev G., O Dnvan F., Szzetti A., 2005, ApJ, 626, 523 Claret, A. 2004, A&A, 428, 1001 Dekker, H. et al. 2000, SPIE, 4008, 534. Deming, D., Seager, S., Richardsn, L. J., & Harringtn, J. 2005, Nature, 434, 740 Fressin, F., Guillt, T., Mrell, V., & Pnt, F. 2007, A&A, 475, 729 Greiner, J., et al. 2008, PASP, 120, 405 Hubeny, I., Lanz, T., & Jeffery, C. S. 1994, SYNSPECa User s Guide, in Newsl. n Analysis f Astrnmical Spectra 20 (St. Andrews Univ.) Knutsn, H. A., et al. 2007, Nature, 447, 183 Liu X., Burrws A., Ibgui L., 2008, ApJ, in press, arxiv: Mandel K. & Agl E. 2002, ApJ, 580, L171 Mandushev et al. 2005, ApJ621, 1061 Mandushev, G., et al. 2007, ApJ, 667, L195 Pasquini, L. et al. 2002, The Messenger 110, 1. Quelz, D., et al. 2001, A&A, 379, 279