Hertzsprung-Russell diagram with single stellar tracks that were used as a basis for the computation of critical mass ratios, minimum orbital separations from SMT evolution, and the corresponding minimum BBH merger delay times (see results for stellar models with semiconvection α_semiconv. = 33 in Fig. 5 and with α_semiconv. = 0.01 in the right panel of Fig. 6). Here there tracks are of 30 different masses, logarithmically spaced from 10 to 100 M_⊙, evolved until at least the end of core-He burning. Black dots are equally spaced every 50,000 years during the post-MS evolution.