We report on high resolution Echelle spectroscopy of 20 giant stars in
the Galactic old open clusters NGC 6791 obtained with Hydra
at the WIYN telescope. High precision radial velocity allow us to
15 bona fide cluster members. From 10 of them we derive a
global [M/H]=+0.39±0.05. We therefore confirm that NGC 6791
is extremely metal rich, exhibits a few marginally sub-solar
abundance ratios, and within the resolution of our spectra does not
show evidences of spread in metal abundance. With these new data we re-derive
the cluster fundamental parameters suggesting that it is about 8 Gyr
old and 4.3 kpc far from the Sun.
The combination of its chemical properties, age, position, and Galactic
orbit hardly makes NGC 6791 a genuine Population I open cluster.
We discuss possible interpretations of the cluster peculiarities
suggesting that the cluster might be what remains
of a much larger system, whose initial potential well could have
been sufficient to produce high metallicity stars, and which
has been depopulated by
the tidal field of the Galaxy. Alternatively, its current
properties may be explained by the perturbation of the Galactic bar
on an object originated well inside the solar ring,
where the metal enrichment had been very fast.