The NGC 6426 RR Lyrae variables and horizontal-branch morphology

We present BVRI CCD photometry for 12 RR Lyrae variables, including three n ewly discovered ones in the Oosterhoff type II globular cluster NGC 6426. N ew light curves and ephemerides are presented. The mean periods of the RRab and RRc variables whose light curves are analyzed in this work are < P-ab> = 0.70 +/- 0.02 days and < P-c> = 0.34 +/- 0.03 days, respectively. The nu mber ratio of the RRe type variables to the total number of RR Lyrae type v ariables is n(c)/n(ab + c)= 0.36. The period-amplitude relation for fundame ntal-mode RR Lyrae variables (RRab) in NGC 6426 supports the recent conclus ion of Clement & Shelton that this relation is not a function of metal abun dance. Fourier decomposition of the light curves has been used to determine the mass, luminosity, and temperature for the RRe stars. Application of th e formula of Jurcsik & Kovacs, which relates Fourier parameters of RRab sta rs to [Fe/H], yielded the value [Fe/H]= -2.16 +/- 0.13 dex, which is compat ible (given the errors) with the value - 2.33 +/- 0.15 dex, derived from th e red giant branch color index developed by Hatzidimitriou et al. From star counts along the horizontal branch (HB), we obtain the Lee et al. HE color distribution index, (B-R)/(B+ V+R) = 0.58 +/- 0.18. The HB of NGC 6426 res embles that of NGC 5053 and M68. We find almost no faint blue stars analogo us to the ones constituting the extended HE in M15.