CCD PHOTOMETRY OF RR LYRAE STARS IN THE GLOBULAR-CLUSTER M68

We present a CCD investigation of the light curves of a sample of RR Lyrae in the metal-poor galactic globular cluster NGC 4590 (=M68). Observations were carried out at ESO in both B and V bands, using the 1.5 m Danish Telescope. Light curves for 21 variables have been derived, giving pulsational amplitude together with mean magnitudes and colors for a subsample of 17 variables. Comparison of the period-amplitude relation with the similar relation for variables in M15 shows a close similarity between the two clusters. The color-period relation supports previous suggestions for a M15 reddening larger than that of M68 by about Delta E(B-V)=0.04. Effective temperatures for M68 pulsators have been derived under selected assumptions about the cluster reddening. Adopting current evaluation for this reddening [E(B-V)similar or equal to-0.03] one finds that the RR Lyrae in this metal-poor globular do not show any period shift to M3, as required by the current evolutionary scenario. The mass-luminosity ratio of the cluster variables is finally discussed, also in connection with recent estimates of such a parameter in M15.