CCD photometry of distant open clusters. II. NGC 6791.

Three new photometric CCD-based datasets are presented for NGC 6791. They consist of deep UBV photometry (to V_lim_=24, B_lim_=24, U_lim_=23) of the central parts of the cluster and of selected fields around it, and of relatively shallower UBVI photometry for the whole cluster (23'x23'). The data have been used to discuss the reddening, metallicity and age of NGC 6791, a cluster which is particularly important because of (1) its most-advanced age among open clusters, (2) metallicity higher than solar and (3) particularly large number of member stars. We discovered two new very blue stars increasing the total of sdB objects in the cluster to ten. They are located in a very narrow range magnitudes V=17.7 +/-0.5; their blue colors strongly constrain our determination of the reddening of the cluster to E(B-V)=0.17 +/-0.01. We detected no other blue stars except a population of faint objects with V=~22 in the center of the cluster. Lack of standard data for clusters with metallicity much higher than solar considerably limited the use of the two-color U-B versus B-V diagram to determine metallicity of the cluster. We have used the new theoretical isochrones in a differential determination of metallicity and age of NGC 6791 using M 67 and NGC 188 as two clusters with well-known abundances and age. We find [Fe/H]=~+0.5 and age by 1 Gyr older than that of NGC 188. The luminosity function of the cluster is SURprisingly flat while the "secondary" main-sequence of equal-mass binaries is weaker than in less-populous old open clusters. We explain both features by the large total mass of the cluster which has led to weak evaporation of low-mass and single stars from the cluster.