Evolution of the gainer’s surface velocity (normalised to the critical velocity) for different spin-down mechanisms for our 6 + 3.6 M_⊙ system with an initial period P_init = 2.5 days. The horizontal line represents the critical spin-angular rotation of the star. All simulations are stopped when Ω_surf ≈ Ω_crit. Top panel: solid black line (labelled “Ref.”): | B_∗ | = 0 kG, no braking mechanism; dotted red line (labelled “B.-L.”): boundary-layer mechanism, no magnetic field; dashed green line: Zahn (1989) tidal effect treatment only; long-dashed blue line: magnetic braking with | B_∗ |  = 3 kG. Bottom panel: solid black line: magnetic braking with | B_∗ | = 1 kG; dotted red line: magnetic braking with | B_∗ | = 2 kG; dashed green line (labelled M.-W.): magnetic-wind braking only with | B_∗ |  = 3 kG; long-dashed blue line (labelled D.-L.): disc locking only with | B_∗ |  = 3 kG.