Model predictions for the ratio of the mass of formed stars to the K-band luminosity (both in solar units) as a function of the B − K colour (in VEGAMAG) of the parent stellar population for different SFH laws, as labelled on top. An absolute magnitude of M_K, ⊙ = 3.26 has been used to compute the mass-to-light ratio of the stellar populations. Filled red circles highlight models with AGE ≥ 12 Gyr in panels a and b, and models with t_U ≥ 13 Gyr in panels c and d. The lines show various options to derive galaxy stellar masses from the B − K colour and K-band luminosity: the dotted line in panel a shows the BJ01 relation for a Salpeter initial mass function (IMF); in all panels the dash-dotted line shows the BJ01-a regression, while the solid and the dashed line are combined in the piece-wise relation. See text for more details.