K_s × (J − K_s) CMDs of NGC 752 (left panel) and NGC 188 (right panel) after the decontamination procedure outlined in this section. The colour bar indicates membership likelihoods according to Eq. (4). Filled (open) symbols represent (non-) member stars, according to our method, and small black dots are stars in a comparison field. Circled filled symbols are stars with photometric membership likelihoods L_phot ≥ 0.1, according to the Maia et al. (2010) decontamination algorithm. The continuous lines are PARSEC isochrones fitted to the data of both clusters, and the dotted lines represent the locus of unresolved binaries of equal-mass components. Dot-dashed lines represent the empirical main sequence of Koornneef (1983) converted into 2MASS according to the relations of Carpenter (2001). Fundamental astrophysical parameters are indicated (see also Table 1).