The NGC 6426 RR Lyrae variables and horizontal-branch morphology

We present BVRI CCD photometry for 12 RR Lyrae variables, including three newly discovered ones in the Oosterhoff type II globular cluster NGC 6426. 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 〈P_ab〉 = 0.70 ± 0.02 days and 〈P_c〉 = 0.34 ± 0.03 days, respectively. The number ratio of the RRc type variables to the total number of RR Lyrae type variables is n(c)/n(ab + c) = 0.36. 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 & Kovács, 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 + V + R) = 0.58 ± 0.18. The HB of NGC 6426 resembles that of NGC 5053 and M68. We find almost no faint blue stars analogous to the ones constituting the extended HB in M15.