Evidences of tidal distortion and mass loss from the old open cluster
  NGC 6791
  We present the first evidence of clear signatures of tidal distortions in the
density distribution of the fascinating open cluster NGC~6791. We find that the
2D density map shows a clear elongation and an irregular distribution starting
from $\sim 300^{\prime\prime}$ from the cluster center and two tails extending
in opposite directions beyond the tidal radius. These features are aligned to
both the absolute proper motion and to the Galactic centre directions.
Accordingly we find that both the surface brightness and star count density
profiles reveal a departure from a King model starting from
$\sim600^{\prime\prime}$. These observational evidences suggest that NGC~6791
is currently undergoing mass-loss likely due to gravitational shocking and
interactions with the tidal field of the Milky Way. We derive the expected
mass-loss due to stellar evolution and tidal interactions and we estimate the
initial cluster mass to be $M_{ini} = (1.5-4.0 ) \times 10^5 M_{\odot}$.
