Profiles of the abundances of H, ³He, ¹²C, ¹³C, ¹⁴N, ¹⁶O and of the mean molecular weight gradient ∇_μ as a function of mass coordinate. The full line stands for ∇_μ > 0 and the plus signs otherwise. Left and right panels correspond to the situation before and after the bump, respectively, for a 0.9   M_⊙ model with 1 × 10^-3. The abscissa ranges from the bottom of the hydrogen-burning shell to the base of the convective envelope.