Evolution of donor stars of different masses in the HR diagram, derived from binary stellar evolution tracks computed with MESA. Nonrotating single stellar tracks from Klencki et al. (2020) are plotted with thin gray lines. The binary models assume a mass ratio of q = M_don/M_acc = 5.0 (3.5 in the case of the M_don = 16 M_⊙ model), and the initial orbital periods were chosen such that the mass-loosing donors are consistent with the location of the AT 2018bwo progenitor in the diagram, albeit for different mass transfer rates.