A New Look at the Old Star Cluster NGC 6791

We present comprehensive cluster membership and g'r' photometry of the prototypical old, metal-rich Galactic star cluster NGC 6791. The proper-motion catalog contains 58,901 objects down to g'~ 24, limited to a circular area of radius 30'. The highest precision of the proper motions is 0.08 mas yr–1. Our proper motions confirm cluster membership of all main and also some rare constituents of NGC 6791. The total number of probable cluster members down to g' = 22 (MV ~ +8) is ~4800, corresponding to M tot 5000 M ☉. New findings include an extended horizontal branch in this cluster. The angular radius of NGC 6791 is at least 15' (the effective radius is Rh 44 while the tidal radius is rt 23'). The luminosity function of the cluster peaks at 4.5 and then steadily declines toward fainter magnitudes. Our data provide evidence that differential reddening may not be ignored in NGC 6791.