BVRI CCD PHOTOMETRY OF THE GLOBULAR-CLUSTER M55 (NGC 6809)

We present BVRI photometry of two neighboring fields in the low concentration globular cluster M55 (NGC 6809) observed with a CCD camera and the 2.2 m telescope at ESO/La Silla. We compare our BV color-magnitude diagram (CMD) with those of Lee [A&AS, 29,1 (1977) ] and Schade et al. [AJ, 96, 1632 (1988)] and find generally good agreement down to about a magnitude below the main-sequence turnoff. We adopt a distance modulus of (m - M)v = 14.10, and find that the fits between our four-color observations and the isochrones published by VandenBerg & Bell [ApJS, 58, 561 (1985)] consistently yield an age of 14-15 Gyr, depending on initial assumptions. Our derived values of interstellar reddening for the various colors are consistent with what would be expected if E (B - V) = 0.16 +/- 0.03. In the four colors, the main-sequence turnoff comes at an average of V(TO) = 17.89 +/- 0.10, with colors at B - V = 0.50 +/- 0.03, V - R = 0.32 +/- 0.02, V - I = 0.70 +/- 0.03, and B - I = 1.20 +/- 0.04. We find that DELTA-V(TO - HB) = 3.54 +/- 0.14, typical of values summarized by Peterson [PASP 99, 1153 (1987) ] for other globulars.