Kippenhahn diagrams with initial 13 M_⊙ early Case B systems accreting (a) 10% and (b) 100% of their initial mass. The moment of mass accretion is indicated by an arrow. Energy generation by nuclear burning, ϵ_nuc, and loss by neutrinos, ϵ_ν, are colour-coded. The right y-axes show the mass fractions of carbon, X_C, and oxygen, X_O, in the centre of the stars. Carbon starts being produced when core helium burning begins. The grey hatched areas show convective regions, purple hatchings are for convective overshooting, yellow stands for thermohaline mixing and brown is for semi-convection. The dotted blue lines show where the hydrogen mass fraction drops below 0.5, that is they trace the helium core mass. Hydrogen shell burning in the f_acc = 0.1 model is radiative and the star evolves into a CSG. In the f_acc = 1.0 model, hydrogen shell burning proceeds convectively and the star evolves as a BSG (cf. Fig. 1).