Functional dependence of ζ_RL on the binary mass ratio q_i ≡ m_d, i/m_a, i, compared to the adiabatic index, ζ_ad, for donor stars of 32 M_⊙ (top panel) and 2 M_⊙ (bottom panel) of different radii. The different colors of the solid lines show the conservative limit, ϵ = 1 (golden), and the completely non-conservative cases of purely isotropic re-emission, β = 1 (dark blue), and mass outflow from L₂ (light blue), see the discussion in Sect. 2.6. Critical mass ratios, ζ_ad, for the MT to be unstable, as calculated by Ge et al. (2020), are shown as horizontal lines (labeled by the donor star radius). For each radius and mode of MT, the critical mass ratio corresponds to the crossing points between ζ_ad and ζ_RL.