Evolution of the mass transfer rate (panel a) and relative Roche lobe overflow (panel b) for a representative 5 M_⊙ donor star losing mass to an initially 1.375 M_⊙ companion with an initial period of about 7 days. The colours and styles of the curves correspond to different mass-transfer efficiencies, as indicated in the legend. Vertical lines indicate the donor-mass at which the quasi-adiabatic instability criterion (see Temmink et al. 2023) is reached in our MESA simulations (with numerical setup as in Section 3). Mass transfer is stable in the fully conservative case.