HR diagram showing evolution of luminosity and surface temperature of donor and accretor star from a binary evolution model with initial donor (M_d, i) and accretor masses (M_a, i) of 79.4 M_⊙ and 19.8 M_⊙, respectively, and an initial orbital period of 15.8 d. Dots are placed along the tracks every 50 000 yr. Coloured lines and symbols (red for the donor and blue for the accretor) are used as long as the donor undergoes core hydrogen burning. The remaining evolution of both stars is shown in grey. The zero-age main sequence is shown as a dashed grey line. The red triangle and blue diamond indicate the position of the WR star and its companion in BAT99#113, respectively, with their error bars.