Hertzsprung-Russell diagram (left panel) and mass–radius evolution (right panel) of the donors of the three models shown in Fig. 1, namely model 1 (dot-dashed black lines), model 2 (short-dashed blue lines) and model 3 (solid green lines). Following the onset of mass transfer, the donor luminosity, effective temperature and radius decrease. The radius of donors with relatively massive helium cores (≳0.05 M_⊙) at the onset of mass transfer eventually starts to decrease faster during the evolution. In case the effective temperature reaches sufficiently high values, the convective envelope vanishes which causes magnetic braking to become inefficient and the donor to detach from its Roche-lobe (illustrated by the magenta circles for model 3). The more evolved the donor at the onset of mass transfer, the higher the donor mass at the moment the radius starts to decrease faster and the higher the maximum effective temperature reached during the evolution.