Kippenhahn diagrams of core hydrogen and core helium burning of stars with an initial mass of 17 M_⊙. The evolution of a genuine single star (panel a) is contrasted with that of a star that underwent (late) Case B mass transfer (panel b). The blue colour-coding shows energy production by nuclear burning, and the green, yellow, purple, and red hatched regions denote convection, thermohaline mixing, convective overshooting, and semi-convection, respectively. The blue and red dotted lines indicate approximate helium and carbon cores, here defined as the mass coordinate where the helium and carbon mass fractions first exceed 0.5.