Values of T_eff as a function of the (R – I)_c color for the data of B12 (filled circles) and M15 (open circles) and a smoothing of these data (solid line). Also shown for comparison: the calibration of Kenyon & Hartmann (1995). This later calibration agrees well with the more recent measures of Mann et al. (2015) up to (R – I)_c = 1.3, but tends to overestimate T_eff for later spectral types. We overplot the (R – I)_c domains of our samples of stars from mid-K to dM7.