Structural changes in the donor star of a DDI binary system, shown for the same representative DDI system as in Figure 1: a donor star with a ZAMS mass of 5 M_⊙ that fills its Roche lobe at a size of 14.3 R_⊙ and loses mass to an initially 1 M_⊙ companion. From left to right, the panels show how the internal luminosity, specific energy generation due to H and He recombination, and shortest timescale vary throughout the donor star during the mass transfer. For visual clarity, negative values (which have a minimum of about −2 ⋅ 10⁶ ergs/s/g) are not shown in panel b. The total mass of the donor star M_d decreases from left to right along the horizontal axis. The vertical axis shows the amount of mass exterior to a shell at mass coordinate m, relative to the total donor mass. In this coordinate system, the core of the star lies towards the bottom of the figure, and the envelope towards the top. Lines of constant log ρ are drawn in white and dashed, with the corresponding values in cgs units indicated on the right hand side of each panel. The solid red line shows the location of the Roche lobe equivalent volume radius inside the star. The vertical green lines and associated letters correspond to the moments shown in Figure 1 and described in Table 1 and the text.