A NEW VISIT TO THE VARIABLE STARS IN M56 AND ITS COLOUR-MAGNITUDE DIAGRAM STRUCTURE

We present a VI CCD photometric study and a membership analysis of the globular cluster M56 (NGC 6779). This produced a CMD decontaminated from field stars, which enabled a better confrontation with theoretical isochrones, zero-age horizontal branches (ZAHB) and post-ZAHB evolutionary tracks. Post He-flash evolutionary models with a He-core mass of 0.5 M-circle dot and envelopes of 0.04 -0.18 M-circle dot, cover the complete horizontal branch. Models with total mass approximate to 0.68 M-circle dot explain the RR Lyrae, while those with a mass approximate to 0.56 Me and a very subtle envelope, explain the Pop II cepheids with a progenitor in the blue tail of the HB. Based on the Fourier decomposition of the V light curve of a single cluster member RRc star, we determined a metallicity of [Fe/H]ZW = -1.96 +/- 0.09. Several independent distance determination approaches lead to a mean distance to M56 of < d > = 9.4 +/- 0.4 kpc. Finally, we report 5 new variables: one SX Phe, three EB, and one RRc.