He abundance (upper panel) and the Rosseland opacity (lower panel) in terms of the outer mass fraction coordinate, corresponding to models with M_⋆ = 0.1868 M_⊙ with element diffusion and different (increasing) values of T_eff, which are marked in Fig. 13. Solid curves correspond to models in which there is pulsation instability, while dashed curves are associated with models in which instability has ceased. The tops of the bumps in the opacity due to the He⁺ −He⁺⁺, He−He⁺, and H−H⁺ partial ionization regions are connected with thin dotted lines, to show their evolution.