Recombination energy in the envelope as a function of ZAMS mass. The plot shows the maximum contribution of recombination energy (from H, He and H₂) to the total internal energy (U) obtained during the evolution of the stars. These maximum values are always reached when the stars are near their largest radial expansion as giants. The red arrows are for Z = Z_MW and the blue ones for Z = Z_⊙/ 50. The green arrows are also calculated for our Z = Z_⊙/ 50 models, but assume a larger remaining core that includes 1M_⊙ of hydrogen.