Critical mass ratios, ﹩{\tilde{q}}_{\mathrm{ad}}﹩, for the onset of dynamical timescale mass transfer as derived from modified evolutionary models with isentropic surface convection zones, in the mass–radius diagram. These models mimic the effects of rapid thermal relaxation in the outer layers of convective stellar envelopes by suppressing the destabilizing effect superadiabatic expansion, thus providing more realistic estimates of the critical mass ratios than the models shown in Figure 6.