The photospheric factor as a function of, from top to bottom, B−V, I−K, and V−K. Dots and crosses represent R_phot for M and GK-dwarfs, respectively. The solid lines are from Eq. (11) for each one of the color indexes. In the top panel, we compare our relationship for the B−V colors to the ones of Hartmann et al. (1984), Noyes et al. (1984), and Mittag et al. (2013, only from synthetic spectra) in dotted, dashed, and dash-dotted lines, respectively. Their range of validity is 0.44 <B−V< 0.82 for Hartmann et al. (1984) and Noyes et al. (1984), and 0.44 <B−V< 1.6 for Mittag et al. (2013).