Same as Fig. 6 but comparing the final orbital separations instead of the BBH delay times. In the variation with low efficiency of semiconvection (right) all the post-MS donors are of the HG type. They are characterized by wider post-SMT separations compared to core-He burning (CHeB) donors due to differences in their internal chemical / entropy structure (Fig. 11). Based on that, we propose a new mass transfer stability treatment in rapid binary codes in Sect. 4.5. The difference between stellar models in both panels is closely connected to the so-called blue supergiant problem (Sect. 4.3).