We present improved values of the three components of the absolute space velocity of the open cluster NGC 6791 .
One HST ACS/WFC field with two-epoch observations provides astrometric measurements of objects in a field containing the cluster center .
Identification of 60 background galaxies with sharp nuclei allows us to determine an absolute reference point , and measure the absolute proper motion of the cluster .
We find ( \mu _ { \alpha } \cos { \delta } , \mu _ { \delta } ) _ { J 2000.0 } = ( -0.57 \pm 0.13 , -2.45 \pm 0.12 ) mas yr ^ { -1 } , and adopt V _ { rad } = -47.1 \pm 0.7 km s ^ { -1 } from the average of the published values .
Assuming a Galactic potential , we calculate the Galactic orbit of the cluster for various assumed distances , and briefly discuss the implications on the nature and the origin of this peculiar cluster .