CCD PHOTOMETRY OF DISTANT OPEN CLUSTERS .2. NGC-6791

Three new photometric CCD-based datasets are presented for NGC 6791. T hey 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' x 23'). The data have been used to discuss the reddening, metall icity and age of NGC 6791, a cluster which is particularly important b ecause of (1) its most-advanced age among open clusters, (2) metallici ty higher than solar and (3) particularly large number of member stars . We discovered two new very blue stars increasing the total of sdB ob jects in the cluster to ten. They are located in a very narrow range m agnitudes V = 17.7 +/- 0.5; their blue colors strongly constrain our d etermination 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 objec ts with V approximate to 22 in the center of the cluster. Lack of stan dard data for clusters with metallicity much higher than salar conside rably limited the use of the two-color U - B versus B - V diagram to d etermine metallicity of the cluster. We have used the new theoretical isochrones in a differential determination of metallicity and age of N GC 6791 using M 67 and NGC 188 as two clusters with well-known abundan ces and age. We find [Fe/H]similar or equal to +0.5 and age by 1 Gyr o lder than that of NGC 188. The luminosity function of the cluster is s urprisingly flat while the ''secondary'' main-sequence of equal-mass b inaries 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.*