2015ApJ...811...46A

We extend our effort to calibrate stellar isochrones in the Johnson-Cousins () and the 2MASS () filter systems based on observations of well-studied open clusters. Using cool main-sequence (MS) stars in Praesepe, we define empirical corrections to the Lejeune et al. color-effective temperature () relations down to, complementing our previous work based on the Hyades and the Pleiades. We apply empirically corrected isochrones to existing optical and near-infrared photometry of cool () and metal-rich () MS stars in NGC 6791. The current methodology relies on an assumption that color- corrections are independent of metallicity, but we find that estimates of color excess and distance from color-magnitude diagrams with different color indices converge on each other at the precisely known metallicity of the cluster. Along with a satisfactory agreement with eclipsing binary data in the cluster, we view the improved internal consistency as a validation of our calibrated isochrones at super-solar metallicities. For very cool stars (), however, we find that colors of our models are systematically redder than the cluster photometry by ∼0.02 mag. We use color- transformations from the infrared flux method and alternative photometry to examine a potential color-scale error in the input cluster photometry. After excluding photometry of these cool MS stars, we derive, [M/H],, and the age of 9.5±0.3 Gyr for NGC 6791.