Evolution of the intrinsic luminosity (L_⋆, solid line), the energy production rate by thermonuclear reaction (L_nuc, dashed) and the seventh part of the gravitational energy of the accreting material (, dotted). The shaded regions and labels from (I) to (V) are the same as in Fig. 1. During phase (II), L_nuc exceeds and the star expands. The flat L_nuc during phase (III) results from the steady and instantaneous fusion of the newly accreted deuterium.