A SEARCH FOR DETACHED ECLIPSING BINARY-SYSTEMS IN THE OLDEST KNOWN OPEN CLUSTER NGC-6791

From 14 consecutive nights of CCD photometry of the central 5.6 x 5.6 arcmin(2) field of the open cluster NGC 6791, we have identified 11 va riable stars, four of these being new variables. Combined with our ear lier four-night survey, these data provide ephemerides and complete li ght curves for three contact binaries and an RS CVn-type detached bina ry. We have identified three more detached binaries, a cataclysmic var iable which underwent an outburst lasting 3 d during the 14-night sequ ence, and three variables which may be spotted stars. Whilst two of th e contact binaries may not be members of the cluster, the remaining bi naries are strong candidates for membership; they are located near the cluster turn-off region and are of astrophysical importance as indepe ndent tests of cluster age and distance. Spectroscopic observations ar e required to establish the masses, absolute dimensions, and distances for these binaries, The colour-magnitude diagram of the cluster is de fined by 2248 stars with photometry accurate to 0.01-0.02 mag, and sho ws a well-defined main sequence from V = 22 mag to the turn-off region at V = 17.3 mag. The subgiant/giant branch extends to V = 15 mag, and there are a substantial number of blue stragglers.