CCD AND IR-ARRAY PHOTOMETRY OF GALACTIC GLOBULAR-CLUSTERS .1. BVJK PHOTOMETRY OF M69

We present new, high-quality optical (B and V) and near-infrared (J an d K) imaging photometry of about 1500 giant and horizontal branch star s in the low-latitude, metal-rich cluster M69 (NGC 6637). With these d ata we find the horizontal branch to be at V(HB) = 15.85 +/- 0.05 (0.3 mag brighter than in previous studies) and at K(HB) = 13.40 +/- 0.05, consistent with the results of Davidge & Simons. The cluster metallic ity, as determined from different properly calibrated observables meas ured on various colour-magnitude diagrams, is found to be [Fe/H] -0.75 +/- 0.20, confirming the strong similarity with 47 Tuc, whose main lo ci nicely fit the M69 data if one adopts a differential distance modul us of 1.40 mag. There is no photometric evidence to support a solar me tallicity as apparently suggested by some integrated-light spectroscop ic features. The brightest giants in our sample are all variable and, after a revision of the distance modulus, they fit the scenario descri bed by Frogel & Elias concerning the core He flash luminosity and vari ability periods. The so-called 'RGB bump' is detected at V = 16.37 +/- 0.05 and K = 13.55 +/- 0.05, in agreement with the results obtained b y Fusi Pecci et al. for about a dozen clusters of various metallicitie s. The proper consideration of its possible influence on the star coun ts used to obtain estimates of the helium abundance via the so-called R-method seems able to explain the lower-than-primordial helium conten t claimed for some metal-rich clusters, yielding Y(p) approximately 0. 23 +/- 0.04, as obtained for other well-observed clusters.