Binary stars selection. Panel a: illustration of the binary-selection procedure. The region enclosed by the azure solid line, dubbed region A, contains all MS stars and binary stars with a primary mass between 0.80 and 1.31 M_⊙. The gray-shaded region, i.e., region B, defines the locus of binary-star candidates with a mass ratio greater than 0.6, respectively. Finally, the dashed black line and red solid lines represent the fiducial line of MS stars and binary stars with mass ratio q = 0.6, as indicated in the top-right legend. Panel b: MS and binary stars identified in the cluster field of NGC 2099, marked with red and blue crosses, respectively. Panel c: same as panel b but for stars located in the reference field, well outside the cluster radius. Stars that fall into region A are marked with black crosses.