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 × 5.6 arcmin² field of the open cluster NGC 6791, we have identified 11 variable stars, four of these being new variables. Combined with our earlier four-night survey, these data provide ephemerides and complete light curves for three contact binaries and an RS CVn-type detached binary. We have identified three more detached binaries, a cataclysmic variable which underwent an outburst lasting 3 d during the 14-night sequence, and three variables which may be spotted stars.Whilst two of the contact binaries may not be members of the cluster, the remaining binaries are strong candidates for membership; they are located near the cluster turn-off region and are of astrophysical importance as independent tests of cluster age and distance. Spectroscopic observations are required to establish the masses, absolute dimensions, and distances for these binaries.The colour–magnitude diagram of the cluster is defined by 2248 stars with photometry accurate to 0.01–0.02 mag, and shows 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.