We extend our effort to calibrate stellar isochrones in the Johnson-Cousins ( BVI _ { C } ) and the 2MASS ( JHK _ { s } ) 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 ( T _ { eff } ) relations down to T _ { eff } \sim 3600 K , 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 ( T _ { eff } \lesssim 5500 K ) and metal-rich ( [ Fe/H ] = +0.37 ) MS stars in NGC 6791 .
The current methodology relies on an assumption that color- T _ { eff } 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 ( T _ { eff } \lesssim 4800 K ) , however , we find that B - V colors of our models are systematically redder than the cluster photometry by \sim 0.02 mag .
We use color- T _ { eff } transformations from the infrared flux method ( IRFM ) and alternative photometry to examine a potential color-scale error in the input cluster photometry .
After excluding B - V photometry of these cool MS stars , we derive E ( B - V ) = 0.105 \pm 0.014 , [ M/H ] = +0.42 \pm 0.07 , ( m - M ) _ { 0 } = 13.04 \pm 0.08 , and the age of 9.5 \pm 0.3 Gyr for NGC 6791 .