Origin of the separation limit lies in delayed dynamical instability. Top: Mass transfer rate (Ṁ) in systems with a 30 M_⊙ donor and a varying mass ratio at RLOF, shown as a function of the decreasing envelope mass. The donor is the same for each mass ratio at the onset of RLOF: ∼250 R_⊙ core-He burning giant with a ∼17 M_⊙ envelope and ∼13 M_⊙ core. Models reaching log(Ṁ/M_⊙ yr⁻¹) > 0.5 become unstable (Sect. 2.4), as indicated by the dashed horizontal line. The critical mass ratio for stability is q_crit = 6.06 (marked in bold). The threshold between stable and unstable models is determined by the Ṁ peak at M_env ≈ 2.5 M_⊙. Middle: Internal entropy profiles of the donor in the q = q_crit model, ranging from the onset of RLOF (light, left) to the end of mass transfer (dark, right). The Ṁ peak seen in the top panel is caused by stripping off the layers where the entropy profile is nearly flat. Bottom: The same entropy profiles but as a function of the radius coordinate. The flat entropy layers are within the inner 3 R_⊙ of the donor (6 R_⊙ during the Ṁ peak), causing instability in models in which the mass transfer would shrink the orbit below a ≲ 8 R_⊙.