  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 ($23' \times 23'$).
The data have been used to discuss the reddening, metallicity and age
ofNGC~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 $V=17.7 \pm 0.5$; their blue colors
strongly constrain our determination of the reddening of the cluster to
$E(B-V)=0.17 \pm 0.01$. We detected no other blue stars except a population of
faint objects with $V \approx 22$ in the center of the cluster. We have used
the new theoretical isochrones in a differential determination of metallicity
and age of NGC~6791 using M67 and NGC~188 as two clusters with well-known
abundaces and age. We find $[Fe/H] \simeq +0.5$ and age by 1~Gyr older than
that of NGC~188. The luminosity function of the cluster is surpisingly 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 lead to weak evaporation of low-mass and single
stars from the cluster.
