Structural changes in the donor star of the same representative DDI system as in Figures 1 and 2. Panel a shows the structure of the donor star in the logρ − logT (in cgs units) plane at different moments in the evolution. Panels b and c show how, respectively, the specific entropy (per baryon) and internal luminosity vary throughout the donor star, using the fractional exterior mass as horizontal coordinate. In panel a, gray dashed lines correspond to adiabats. In panels b and c, gray shading indicates where the star is convective. In all panels, the lines are coloured and labelled according to the moments in the legend (see also Table 1). Crosses indicate the location of the Roche lobe equivalent volume radius inside the star.