The absolute motion of the peculiar cluster NGC 6791

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))(J2000.0) = (-0.57 +/- 0.13, -2.45 +/- 0.12) mas yr(-1), and adopt V-rad = -47.1 +/- 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.