Deep CCD photometry in globular clusters. VI - White dwarfs, cataclysmic variables, and binary stars in M71

In a search for white dwarfs in the globular cluster M71, the inner 6' x 4' of the cluster was imaged in U, B, and V at CFHT. Stars fainter than 25th magnitude are recorded in all three colors. A sequence of faint blue stars was detected, but, at a given color, the stars are somewhat fainter than expected if their properties are similar to field DA while dwarfs. Their colors and magnitudes are much better matched by a sequence consisting of DB stars or by a sequence ∼ 0.1 Msun more massive than the canonical mass for disk white dwarfs. Ar unexpected result was the discovery of a very blue sequence of stars which appear to be much too luminou to be single white dwarfs. Their colors and magnitudes are consistent with their identification as cluster cataclysmic variables or as foreground white dwarfs, but the available statistics are incapable of deciding between these two options at present. Low-resolution spectroscopy, searching for Balmer emission lines could, however, be definitive in choosing between these possibilities. Finally, except possibly for a sample of blue stragglers which number more than 50 (∼ 1 % of all stars measured), the binary frequency in the cluster center is low.