Left panel: Kippenhahn diagram showing the interior structure of the 12M_⊙ primary star of the reference model over time. We show nuclear burning regions shaded in blue, convection in hatched green and overshooting in crosshatched purple. The solid black line shows the surface of the star, which decreases quickly when the envelope stripping occurs. The letters denote the same evolutionary phase as labeled in Fig. 2. Right panel: corresponding Kippenhahn diagram for the lower metallicity (Z = 0.0002) model. There is a larger amount of leftover hydrogen-rich envelope after Roche-lobe overflow.