the metal abundance of the cluster (from RRab stars). Finally, in 5 we discuss the HB morphology of NGC 6426.

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1 THE ASTRONOMICAL JOURNAL, 1:851È858, 2000 February ( The American Astronomical Society. All rights reserved. Printed in U.S.A. THE NGC 6426 RR LYRAE ARIABLES AND HORIZONTAL-BRANCH MORPHOLOGY I. PAPADAKIS,1 D. HATZIDIMITRIOU,1 B. F. W. CROKE,2 AND I. PAPAMASTORAKIS1,2 Received 99 September 3; accepted 99 October ABSTRACT We present B RI CCD photometry for 12 RR Lyrae variables, including three newly discovered ones in the Oosterho type II globular cluster NGC New light curves and ephemerides are presented. The mean periods of the RRab and RRc variables whose light curves are analyzed in this work are SP T \ 0.70 ^ 0.02 days and SP T \ 0.34 ^ 0.03 days, respectively. The number ratio of the RRc type variables ab to the total number of RR c Lyrae type variables is n(c)/n(ab ] c) \ The period-amplitude relation for fundamental-mode RR Lyrae variables (RRab) in NGC 6426 supports the recent conclusion of Clement & Shelton that this relation is not a function of metal abundance. Fourier decomposition of the light curves has been used to determine the mass, luminosity, and temperature for the RRc stars. Application of the formula of Jurcsik & Kova cs, which relates Fourier parameters of RRab stars to [Fe/H], yielded the value [Fe/H]\[2.16 ^ 0.13 dex, which is compatible (given the errors) with the value [ 2.33 ^ 0.15 dex, derived from the red giant branch color index developed by Hatzidimitriou et al. From star counts along the horizontal branch (HB), we obtain the Lee et al. HB color distribution index, (B[R)/(B] ]R) \ 0.58 ^ 0.. The HB of NGC 6426 resembles that of NGC 5053 and M68. We Ðnd almost no faint blue stars analogous to the ones constituting the extended HB in M15. Key words: globular clusters: individual (NGC 6426) È RR Lyrae variable 1. INTRODUCTION NGC 6426 is one of the most metal-poor globular clusters ([Fe/H] \[2.33 dex) in the Galactic halo. Zinn and Barnes (96) constructed the Ðrst color-magnitude diagram for the cluster. They found that the horizontal branch (HB) of the cluster is typical of the very poor globular clusters in being blue, but not extremely blue. In particular, they found that it lacks the long blue extension displayed by other metal-poor clusters (such as M15). Recently, Hatzidimitriou et al. (99, hereafter Paper I), constructed a deeper color-magnitude diagram. Their work yielded improved measurements of the metal abundance, reddening, and distance modulus of the cluster. Furthermore, they found that the age of NGC 6426 is similar to the mean age of the most metal-poor clusters in the Galactic halo. The Ðrst and only existing study of the RR Lyrae variables of NGC 6426 was done by Grubissich (58, hereafter G58). In this work we present a new photometric study in B RI of 12 RR Lyrae variables, including three newly discovered ones in this cluster. We compare our results with those of other clusters of similar metal abundance. In particular, our results from the period versus -amplitude relation for the RRab stars in NGC 6426 strongly support the result of Clement & Shelton (99a, hereafter CS99) that the amplitude for a given period is not a function of metallicity. In 2, we describe brieñy the observations and reductions, and in 3 we present the light curves and periods of the RR Lyrae variables found. In 4 we derive Fourier decomposition parameters from which we deduce the mass, luminosity, and temperature of the RRc stars and ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Department of Physics, University of Crete, P.O. Box 2208, GR , Heraklion, Crete, Greece. 2 Foundation for Research and TechnologyÈHellas, P.O. Box 1527, GR , Heraklion, Crete, Greece. 851 the metal abundance of the cluster (from RRab stars). Finally, in 5 we discuss the HB morphology of NGC OBSERATIONS AND REDUCTIONS NGC 6426 was observed, through standard B,, R, and I Ðlters, on several di erent runs during 95 and through and R Ðlters on several nights during 97 using the 1.3 m telescope at Skinakas Observatory. A log of the observations, together with details about the telescope, the CCD camera, and the reduction of the observations can be found in Paper I. In total, there are 35 B frames, 96 frames, 70 R frames, and 55 I frames. Each frame was analysed using the pointspread function (PSF) Ðtting routines of DAOPHOT. The DAOPHOT magnitudes obtained for each star were related to the standard-star observations by using aperture photometry on 35 bright, uncrowded stars to determine the o sets between the PSF instrumental magnitudes and magnitudes on the same instrumental system as the standard-star observations, as described in Paper I. 3. RR LYRAE PHOTOMETRY The apparent magnitude of the HB in the middle of the instability strip is D mag for NGC 6426 (Zinn & Barnes 96). Since we are interested in the RR Lyrae variables, we have limited our search to stars with magnitudes within the range 16.7 mag \ \.5 mag. There are 395 such stars. We have computed weighted mean values and standard deviations about the mean, p. (For the calculation of the weighted mean values we used s the photometric errors of the magnitudes returned by ALLSTAR.) Since in a crowded Ðeld large scatter can be introduced by incorrect measurements of blended stars, we calculated the residuals about the mean, discarded the two most deviant values, and recalculated the mean and standard deviation. We also computed the expected ÏÏ scatter about the mean for each star, p \ n p /n, where p is the total photometric error (including e i/1 the i PSF-Ðtting i error returned by ALLSTAR and the errors introduced from the transformation of the instru-

2 852 PAPADAKIS ET AL. ol. 1 mental magnitudes to the standard photometric system), and n is the number of measurements for each star. Our list of RR Lyrae candidate stars was obtained by selecting all the stars with p [ 2.5p in all four wave bands (B,, R, and I). There are 16 s stars that e satisfy this criterion. We then searched for periodic variations using a program based on the Lomb-Scargle periodogram (Lomb 76; Scargle 82). We calculated the periodogram at equispaced frequencies, the lowest of which was the span of the input data, T \ max (t ) [ min (t ). Higher frequencies were integer multiples of D0.002 i day~1, i and the highest frequency that we examined was D5 day~1 (corresponding to a period of 0.2 days). For all stars, we calculated the periodogram for each B,, R, and I light curve. Because of the almost 2 yr gap in the and R light curves, we did not treat all measurements simultaneously but instead calculated the periodogram of the /95, /97, and R/97 light curves independently. We found that in this way we have a more accurate estimation of the period (as indicated by the phased light curves and the agreement with the estimated period from the other wave band light curves). It is noted that, because of seeing variations and consequent image crowding, the R/95 light curves of most of the stars in the sample have too few points (typically less than 20) to allow for accurate estimation of the period, and thus they were not used. As a result of this procedure, we discovered signiðcant peaks in the periodograms of 12 of the 16 stars in the sample. isual inspection of the phased light curves is sufficient to decide the reality of the periodic variations in all of them. The remaining four stars are located in crowded regions of the cluster, and their photometry is probably seriously a ected by blending problems. Ten of the 13 stars identiðed by G58 as RR Lyrae variables lie within the area of the sky that we observed. Nine of these stars were rediscovered. The exception is the G58 variable 3, for which we could not Ðnd any signiðcant period (but see below). We provide period estimates for variables 7 and 8, for which no periods were given in G58. We have discovered another three RR Lyrae variables. We have assigned to these three new variables the names 14, 15, and 16, continuing the numbering in the G58 catalog. A Ðnding chart for the variables is given in Figure 1, while the individual -phased light curves are plotted in Figure 2. The light curves are arranged in order of increasing period. The photometry of the RR Lyrae variables is listed in Table 1 (the ModiÐed Julian Dates correspond to the midpoint of the corresponding band exposures). We also performed a periodogram analysis of the light curves of those stars with p [ 2.5p in three or fewer Ðlters. s e No additional stars showed prominent peaks in their perio FIG. 1.ÈFull CCD Ðeld, 5@.7 ] 5@.7, with north to the left and east at the bottom. All of the RR Lyrae variables listed in Table 2 (including 3) are identiðed (the origin of the pixel numbers is the lower left corner).

3 No. 2, 2000 NGC 6426 RR LYRAES FIG. 2.È -magnitude light curves as a function of phase for the NGC 6426 RR Lyrae variables: 95 observations ( Ðlled circles) and 97 observations (open circles). dograms. This indicates that the variability shown in the individual Ðlter light curves of these stars is due to uncertainties in the photometry measurements of some frames. Table 2 lists the variables, their x and y positions on the CCD frame (in pixels), the Bailey types, the periods, the Ñux-weighted mean B,, R, and I magnitudes, and the amplitudes (those amplitudes were estimated with a simple eyeball ÏÏ Ðt to the -phased light curves). The periods listed in Table 2 are the average values of the periods estimated from the individual /95, /97, B/95, R/97, and I/95 light curves. The period error is equal to the standard deviation of the respective mean periods. We also measured the

4 854 PAPADAKIS ET AL. ol. 1 TABLE 1 PHOTOMETRY OF THE RR LYRAE ARIABLES MJD B p MJD p MJD R p MJD I p B B R R I I ariable Star: 1 49, , , , , , , , , , , , , , , , , , , , NOTE.ÈTable 1 is presented in its entirety in the electronic edition of the Astronomical Journal. A portion is shown here for guidance regarding its form and content. uncertainty in the estimated periods using the, R/95 and the, R/97 phased light curves as follows. Typically, these two data sets span a range of D730 days. Assuming no signiðcant changes in the period of the variables, the phase di erence of the light-curve maxima can be used to give us a direct measurement of the accuracy of the estimated period. This method gave us accuracy measurements that were in good agreement with the errors listed in Table 2. The mean periods of the RRab and RRc variables studied in this work are P \ 0.70 ^ 0.02 days and P \ 0.35 ab c ^ 0.03 days, respectively. We have excluded star 16 from these calculations, as its classiðcation is ambiguous (see below). It is noteworthy that the mean period of the RRab variables is among the longest known. Only NGC 5897 (Wehlau 90) has a larger value of P. However, if we ab include 16 in the RRab category, the mean period becomes P \0.67 ^ 0.04 days, which is similar to that of ab several other Oosterho type II clusters, such as NGC 24, NGC 4833, NGC 5053, and M15. Even so, the period distribution remains unusual, with a marked depletion in the numbers of RRab stars with periods shorter than 0.68 days. In this sense, the period distribution is similar to but less extreme than that of NGC It should be noted that NGC 5897 is signiðcantly more metal-rich than NGC 6426 ([Fe/H]\[1.66 ^ 0.10 dex; Sarajedini 92), indicating that the mean P for each cluster does not depend on ab metallicity. The RR Lyrae distribution by type yields n(c)/ n(ab ] c) \ 0.36 (excluding 16 as before). If we include variables 2, 11, and 13 (which lie outside the area studied here) of the G58 catalog, we derive the same value. The ratio becomes 0.40 if we assume that 16 is an RRc star. This value is somewhat lower than the value 0.49 derived for M15, a cluster with metal abundance similar to that of NGC 6426 (Silberman & Smith 95) Comments on Individual RR L yrae ariables 3: This is listed by G58 as an RRc star with a period of days. Clement & Nemec (90) showed that 3 is in fact a double-mode pulsator with fundamental period P \ 0.54 days and Ðrst overtone period P \ days. 0 Although this is one of the 16 stars in our 1 list that show signiðcant variability, we failed to Ðnd any signiðcant peaks in the periodogram of any of the light curves of the four Ðlters. In Figure 3 we plot the -phased light curve for this star using the Ðrst overtone period (which is equal to the G58 period). The resulting light curve is of poor quality. 3 is signiðcantly brighter than the other RR Lyrae variables in this cluster, and its photometry may be a ected by the nearby stars and should therefore be treated with caution. 4: The period given by G58 is days, and its light curve showed a sinusoidal RRc-type behavior. However, our data show 4 to be an RRab type with period of days, the longest period RR Lyrae variable in the present sample. The old period appears to be an alias of the new. In Figure 3 we show the phased light curve using the old G58 period. 6: The light curve shows a possible Blazhko e ect in the secondary bump. 7: G58 does not give a period estimate for this star. We identify it as an RRab star, with a period of days. The light curve displays the Blazkho e ect. 8: As above, G58 does not give a period estimate for this star. We identify it as an RRab star, with a period of days. The light curve shows a possible Blazkho e ect in the secondary bump. TABLE 2 ELEMENTS OF THE RR LYRAE ARIABLES Period Star x y Type (days) SBT S T SRT SIT A / 31 D m ab 0.66 ^ ^ ^ ^ ^ ^ ab ^ ^ ^ ^ ^ ^ ab ^ ^ ^ ^ ^ ^ ab ^ ^ ^ ^ ^ ^ ab ^ ^ ^ ^ ^ ^ ab ^ ^ ^ ^ ^ ^ c ^ ^ ^ ^ ^ ^ c ^ ^ ^ ^ ^ ^ c ^ ^ ^ ^ ^ ^ c ^ ^ ^ ^ ^ ab 0.69 ^ ^ ^ ^ ^ ^ ab ^ ^ ^ ^ ^ ^

5 No. 2, 2000 NGC 6426 RR LYRAES FIG. 3.È -magnitude light curves of the 12, 9, 4, and 3 variable stars, plotted as a function of phase using the best-ðt period as listed in G58. For clarity, we have plotted only the /95 data set. 9: The period given by G58 was days. The present analysis shows that the new period is days. The old period appears to be an alias of the new. In Figure 3 we show the phased light curve using the old G58 period. The maximum and minimum magnitudes for the di erent epochs of observation do not match well. This suggests that 9 is possibly a double-mode variable. It is also the brightest RR Lyrae variable in the sample, indicating that it is highly evolved. 12: It is the bluest RR Lyrae variable in the sample. The period given by G58 was days. The present analysis shows that the new period is days. The old period appears to be an alias of the new. In Figure 3 we show the phased light curve using the old G58 period. 14: This is a newly discovered RR Lyrae variable in the cluster. It displays an RRc-type light curve, with the lowest amplitude (0.35) and shortest period ( days) of all RRc stars in the sample. It has a poor-quality light curve, with large point-to-point scatter. There appears to be a systematic disagreement between the two observing epochs, indicating that either the period or the amplitude may be changing. 15: This is a newly discovered RR Lyrae variable in the cluster. It is an RRab-type variable, displaying the Blazhko e ect. 16: This is also a newly discovered RR Lyrae variable. The light curve is compatible with that of an RRab displaying the Blazhko e ect. The mean color of the star is consistent with this classiðcation. However, the period found is consistent with a star of the RRc-type (albeit with the largest amplitude of any RRc star in the sample). We have searched for both longer and shorter period modulation, but we have failed to Ðnd any convincing ones. 4. FOURIER DECOMPOSITION We have applied the technique of Fourier decomposition (Simon 88) to the light curves of the RRab and RRc stars. The light curves of the RR Lyrae variables at each Ðlter were Ðtted with a Fourier series of the following form: magnitude \ A ] A cos ( jut ] / ), whereupon coeffi- cients / \ / 0 [ j/ j (for j j \ 1, 2, 3, j 4) were calculated. Errors in j1 the j important 1 phase coefficient / from the Fourier decomposition of the individual light 31 curves were typically D0.2È0.3 in most cases. The only exception is 14. The errors on the parameters from the Fourier decomposition of this star were very large. Therefore, we do not present results for this star. We have calculated the weighted mean of the best-ðt values of /, derived from the respective B,, R, and I light curves. 31 These mean values (together with the corresponding error) are listed in Table Derivation of [Fe/H] from the Fourier Decomposition of RRab Stars Using the parametrization of Jurcsik & Kova cs (96), we can derive [Fe/H] from the Fourier decomposition of

6 856 PAPADAKIS ET AL. ol. 1 RRab stars, according to the equation [Fe/H] \[5.038 [ 5.394P ] 1.345/ 31. (1) Care must be taken that the RRab stars to which this equation is applied follow the light-curve systematics deðned by the calibrating sample of Jurcsik & Kova cs. This can be ensured if the light curve in question satisðes a certain compatibility criterion. The various Fourier parameters for the calibrating data set obey well-deðned correlations (see Table 6 of Jurcsik & Kova cs). Deviation of a light curve from the calibrating characteristic is quantiðed via the deviation parameter D, deðned as D \ o F [ F o /p, where F is the observed F value of F the given obs Fourier calc parameter, F F obs is its predicted value from the other observed parameters, calc and p is the corresponding deviation of the various correlations F (Table 6 of Jurcsik & Kova cs 96). If the maximum value, D, of the D of each parameter is less than 3, then the light m curve satisðes F the compatibility criterion. We have performed this exercise for the light curves of all RRab stars. The last column in Table 2 lists the maximum deviation parameter, D, for each star. Three of the eight m RRab stars, namely, 1, 5, and 6, have D \ 3. Those m are the stars with the most regular light curves, as can be seen in Figure 2. Using the / values from Table 2 for these stars (note 31 that Jurcsik & Kova cs used a sine rather than a cosine Fourier decomposition; therefore the / values have to be 31 increased by n before applying eq. [1]), we derive from equation (1) the following [Fe/H] values: [1.77 ^ 0.20, [1.94 ^ 0., and [1.87 ^ 0.13, respectively, yielding a weighted mean of [1.87 ^ 0.10 dex. This metal abundance is not on the Zinn & West (84) scale. The relation between the metallicity scale used by Jurcsik & Kova cs and the Zinn & West scale is derived by comparing the metallicities derived using their method for six Galactic globular clusters (M4, M5, M68, M92, M107, and NGC 3201) with the corresponding known abundances of these clusters on the Zinn & West scale (data taken from Chaboyer, Demarque, & Sarajedini 96). The resulting relation is very well deðned: [Fe/H] \ (1.028 ^ 0.032)[Fe/H] ZW JK [ (0.242 ^ 0.045). Applying this transformation, the resulting value for NGC 6426 becomes [2.16 ^ 0.13 dex. This is compatible, given the combined errors, with the value [2.33 ^ 0.15 dex found in Paper I from the red giant branch color index Derivation of Mass, L uminosity, and E ective T emperature from the Fourier Decomposition of the L ight Curves of RRc Stars The phase coefficient / for the RRc stars can be used to derive important physical 31 parameters, such as the mass, luminosity, and e ective temperature of the star. The hydrodynamic pulsation models of Simon & Clement (93) yield TABLE 3 QUANTITIES DERIED FROM FOURIER FITTING FOR THE RRc STARS the following equations (for RRc stars): log M \ 0.52 log P [ 0.11/ ] 0.39, (2) 31 log L \ 1.04 log P [ 0.058/ ] 2.41, (3) 31 and log T \ [ log P [ 0.77 log M eff ] log L. (4) In Table 3 we tabulate the values for the mass, luminosity, and e ective temperature for the three RRc stars (namely, 9, 10, and 12, for which it was possible to estimate / ), as derived from equations (2), (3), and (4). The mean mass 31 of the three RRc variables, including 9, which has a somewhat anomalous light curve (see 3.1) is 0.66 ^ 0.11 M. Excluding this, the mean value becomes 0.73 ^ 0.01 M _. This value is very similar to the values found for other _ Oosterho type II clusters for which similar studies have been conducted: for M68 ([Fe/H]\[2.1), SM T \ 0.70 ^ 0.01 M (Walker 94) and, for M15 ([Fe/H]\[2.28), RR SM T _ \ 0.73 M (Kaluzny et al. 98). The mean luminosity RRc (for 10 and _ 12) is log L \ 1.77 L. This is again very similar to that of M68 (log L \ 1.79 _ L ) and of M15 (log L \ 1.80 L ). The mean e ective tem- perature is 7200 ^ 70 K, which is marginally higher than the values found for M68 (7145 K) and M15 (7136 K) and similar to the value found for the more metal-rich cluster NGC 2298 ([Fe/H] \[1.90; Clement, Bezaire, & Giguere 95). Because of the small size of the sample of RRc stars found in NGC 6426, it is not possible to investigate correlations of the various parameters further. Suffice it to note that all values that we could derive are compatible with the Oosterho type and metal abundance of the cluster, thus conðrming the earlier result (Kaluzny et al. 98) that mean masses and mean luminosities increase and mean temperatures decrease with decreasing cluster metal abundance Amplitude versus Period Diagram It has often been assumed (e.g., Sandage 81) that the period-amplitude relation for fundamental-mode RR Lyrae variables (RRab) is a function of metal abundance. Recently, CS99 have reexamined the issue using RRab stars with normal ÏÏ light curves, i.e., with light curves satisfying the D \ 3 criterion of Jurcsik & Kovaks (96) mentioned in 4.1. m They used seven well-studied clusters, four of Oosterho type I (M3, M107, M4, and M5) and three of Oosterho type II (M9, M68, and M92). They found no correlation of the periodè -amplitude relation with metal abundance for either Oosterho type. In Figure 4, we plot the log of the period versus amplitude for the RR Lyrae variables of NGC On the same Ðgure, we mark the location of the RR Lyrae variables of M9 ([Fe/H] \[1.72) with Ðlled circles (data from Clement & Shelton 99b). The agreement is remarkably good, although there is a 0.6 dex di erence in metal abundance between the two clusters. This P log M log L log T eff Star (days) (M _ ) (L _ ) (K) ^ [0.276 ^ ^ ^ ^ [0.131 ^ ^ ^ ^ [0.144 ^ ^ ^ 0.007

7 No. 2, 2000 NGC 6426 RR LYRAES A log P (day 1 ) FIG. 4.ÈDiagram of amplitude (A ) versus log of period for RRab stars with D \ 3 (squares), other RRab stars (crosses), RRc stars (open circles), m and the RRab stars of M9 ( Ðlled circles; from Clement & Shelton 99b). With the diamond we have plotted 16, whose classiðcation is ambiguous (see text). result strongly supports the conclusion of CS99 that the period-amplitude relation for fundamental-mode RR Lyrae variables is not a function of metal abundance. The fact that the RR Lyrae variables in NGC 6426 comply with this statement is particularly interesting, given the very low metal abundance of this cluster. The only cluster of comparable metal abundance in the CS99 sample is M92, which has very few known RR Lyrae variables, thus providing by itself a somewhat weak constraint at the extreme metalpoor end. As CS99 pointed out, this result is particularly important regarding the derivation of cluster ages, using the visual magnitude di erence between the zero-age HB and the main-sequence turno (* ZAHB). TO 5. HORIZONTAL BRANCH MORPHOLOGY Using the average of the mean magnitudes of the RR Lyrae variables listed in Table 2, we found that the apparent magnitude of the HB at the middle of the instability strip, (HB), is.16 ^ Given the errors, this value agrees well with the estimate of Zinn and Barnes (96) of (HB)\.12 ^ These authors had estimated (HB) from the nonvariable stars bordering the instability strip on either side. Note that the value (HB) \.14 ^ 0.02, used in Paper I, was based on preliminary results from the analysis of the RR Lyrae light curves. Nevertheless, it is almost identical to the value found in this work after the complete analysis of the light curves. Figure 5 shows the versus [I diagram of the red giant branch and HB area (the data are taken from Paper I, but the innermost region of 30A radius is excluded for completeness reasons; see below) with the RR Lyrae variables marked as Ðlled circles (RRab) and open circles (RRc). The dereddened color [E( [I) \ 0.53 ^ 0.03 from Paper I] of the blue edge of the instability strip is [I \ 0.35 (mean between bluest RR Lyrae and reddest blue HB nonvariable star), compared with 0.34 for M68. The dereddened color of the red edge is [I \ 0.59, compared with 0.54 for M68 (Walker 94) and 0.58 for M15 (Silbermann & Smith 95). We also computed the index (B[R)/(B] ]R) characterizing the HB morphology (Lee, Deamrque, & Zinn 90), I FIG. 5.ÈColor-magnitude diagram, showing the horizontal branch region, with the RRab stars ( Ðlled circles) and the RRc stars (open circles). where B and R are the numbers of nonvariable HB stars in the blue and red side of the instability strip, respectively, and is the number of RR Lyrae variables. Care was taken to calculate this index beyond the inner radius of 30A, soas to ensure that the completeness is similar for variable and nonvariable HB stars. The value we thus Ðnd is 0.58 ^ 0., similar to the value quoted by Zinn & Barnes (96). This value is characteristic of the HB of other very low metallicity clusters such as M68, NGC 5053, and NGC Finally, in Figure 6 we compare the horizontal branches of three metal-poor clusters, namely, NGC 6426, M68 ([Fe/H] \[2.09 dex, E( [I) \ 0.09, (HB) \ 15.64), and NGC 5053 ([Fe/H] \[2.42 dex, E( [I) \ 0.08, (HB) \ 16.65). The data for NGC 6426 have been taken from Paper I, while data and cluster parameters for M68 and NGC 5053 have been taken from Walker (94) and Sarajedini and Milone (95), respectively. In the construction of Figure 6, we have shifted the NGC 5053 and M68 points vertically and horizontally to match the (HB) and E( [I) of NGC 6426 [ (HB) \.16 and E( [I) \ 0.53]. In this Ðgure, one can see that NGC NGC6426 M68 NGC I FIG. 6.ÈComparison of the horizontal branch and red giant branch region of three metal-poor clusters, NGC 6426 ([Fe/H] \[2.33 dex), M68 ([Fe/H] \[2.09 dex), and NGC 5053 ([Fe/H] \[2.42 dex.

8 858 PAPADAKIS ET AL. is very similar to NGC 5053 and M68. As also noted by Zinn & Barnes (96), NGC 6426 lacks both the blue clump and gap seen in M68 and the faint blue stars that make up the extended ÏÏ HB observed in other metal-poor globular clusters (e.g., M15; Silbermann & Smith 95). Higher spatial-resolution data that could lead to good quality photometry in the crowded inner region of the cluster will resolve the question conclusively. 6. SUMMARY We have presented new CCD photometry of the RR Lyrae variables in the globular cluster NGC Together with the three newly discovered variables, the RR Lyrae population of this cluster consists of one RRd, Ðve RRc, and 10 RRab stars (counting 16 as an RRab star). Fourier decomposition of the RRab light curves and application of the Jurcsik & Kovacs (96) formula yielded the value [Fe/H] \[2.16 ^ This value agrees well (within the errors) with the value [2.33 ^ 0.15 found in Paper I. Comparison between the RRab stars of M92 and NGC 6426 showed that these stars follow the same amplitude-period relationship. With NGC 6426 being signiðcantly more metal-poor than M9, this result strongly supports the result of CS99 that there is no correlation between this relationship and metallicity among Oosterho type II clusters. Finally, as in the results of Zinn & Barnes (96), we also found that the HB of NGC 6426 is typical of metal-poor globular clusters (HB type index \ 0.58 ^ 0.13), lacking the extended, heavily populated, long HB found in M15. This research has been supported by a P.EN.E.D. Program of the General Secretariat of Research and Technology of Greece. D. H. was partially supported by a grant from the American Astronomical Society for this work. Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and TechnologyÈHellas, and the Max-Planck-Institut fu r extra- terrestrische Physik. We are grateful to M. Xilouris, M. Woche, and E. Semkov for helping with the observations. Chaboyer, B., Demarque, P., & Sarajedini, A. 96, ApJ, 459, 558 Clement, C. M., Bezaire, J., & Giguere, D. 95, AJ, 110, 2200 Clement, C. M., & Nemec, J. 90, J. R. Astron. Soc. Canada, 84, 434 Clement, C. M., & Shelton, I. 99a, ApJ, 515, L85 (CS99) ÈÈÈ. 99b, AJ, 1, 453 Grubissich, C. 58, Contrib. Padua Obs. 94 Hatzidimitriou, D., Papadakis, J., Croke, B. F. W., Papamastorakis, I., Paleologou, E.., Xanthopoulos, E., & Haerendel, G. 99, AJ, 1, 3059 (Paper I) Jurcsik, J., & Kova cs, G. 96, A&A, 312, 111 Kaluzny, J., Hilditch, R. W., Clement, C., & Rucinski, S. M. 98, MNRAS, 296, 347 Lee, Y.-W., Demarque, P., & Zinn, R. 90, ApJ, 350, 155 REFERENCES Lomb, N. R. 76, Ap&SS, 39, 447 Sandage, A. 81, ApJ, 248, 161 Sarajedini, A. 92, AJ, 104, 8 Sarajedini, A., & Milone, A. A. E. 95, AJ, 109, 269 Scargle, J. D. 82, ApJ, 263, 835 Silbermann, N. A., & Smith, H. A. 95, AJ, 110, 704 Simon, N. R. 88, ApJ, 328, 747 Simon, N. R., & Clement, C. M. 93, ApJ, 410, 526 Walker, A. 94, AJ, 108, 555 Wehlau, A. 90, AJ, 99, 250 Zinn, R., & Barnes, S. 96, AJ, 112, 1054 Zinn, R., & West, M. J. 84, ApJS, 55, 45