Infrared Photometry of NGC 6791

We present deep JHK photometry of the old and metal-rich open cluster NGC 6791. The photometry reaches below the main sequence turn-off to K = 16.5 mag. We combine our photometry with that from Stetson, Bruntt, & Grundahl (2003) to provide color-magnitude diagrams showing K vs. J-K, K vs. V-K, and V vs. V-K. We study the slope of the red giant branch in the infrared, but find that it is not a useful metallicity indicator for the cluster, nor any metal-rich cluster that lacks a well-populated red giant branch, because it is not linear, as has often been assumed, in K vs. J-K. The mean color of the red horizontal branch/red clump stars provide an estimate the cluster reddening, E(B-V) = 0.14 +/- 0.04 mag for [Fe/H] = +0.4 +/- 0.1. The mean magnitudes of these stars also provide a good distance estimate, (m-M)_0 = 13.07 +/- 0.04. Finally, we find that the isochrones of Yi, Kim, & Demarque (2003) provide optimal fits in V vs. B-V and V-K and K vs. J-K and V-K for such values if [Fe/H] lies between +0.3 and +0.5 (with a slight preference for +0.5) and ages between 9 Gyrs ([Fe/H] = +0.3) and 7.5 Gyrs ([Fe/H] = +0.5).