Binary evolutionary tracks for two stars with a rotational mixing efficiency f_m = 0.66 and with initial masses M_1, i = 36 M_⊙ and M_2, i = 15 M_⊙, and an initial period of 1.55 days. The best fitting positions on the track are indicated with black open circles and correspond to an age of 17 kyr. The 36 M_⊙ star collapses into a BH at an age of 7.8 Myr, indicated also on the 15 M_⊙ track with a blue triangle. After that, the 15 M_⊙ star evolves further until it starts transferring mass to the BH (red square). Eventually, the 15 M_⊙ star will also collapse into a BH (the track stops at carbon depletion).