Evolution of the total mass, the convective core mass, and the helium core of the donor model with an initial mass of 20 M_⊙ with a companion of initially 14 M_⊙ and an initial orbital period of 10^0.35 d = 2.2 d (top panels) and 10^0.75 d = 5.6 d (bottom panels). We indicate the mass of the convective core at the onset of mass transfer, just after the fast Case A, and at central hydrogen depletion by grey lines.