M_bol as a function of M_V (filled color circles connected with a line, the colors being related to stellar mass according to the colorscale shown on the right of the figure), from isochrones at log (t) = 7.4 (t being the time in yr) published by Georgy et al. (2014), in the stellar mass range from 2.4 M_⊙ to 3.6 M_⊙. The derivative dM_bol/ dM_V computed numerically between two adjacent data points is plotted with + symbols. A dotted line is drawn at the Y-axis value of 0.6, which is the value adopted for this derivative in the analytical considerations presented in the Letter.