Evolution of the region between the HBS and the convective envelope, when the prescription of Kippenhahn et al. (1980) is adopted to compute the diffusion coefficient in the thermohaline zone. The time interval spans from the instant when the stars luminosity reaches L ≈ 96 (i.e., before the luminosity bump) until L ≈ 1826, close to the top of the RGB. The figure corresponds to the 0.9 M_⊙, Z = 1 × 10^-3, model.