Profile of the mean molecular weight gradient ∇_μ as a function of mass. Solid line stands for ∇_μ> 0 regions and the dashed line for all other cases. Left and right panels correspond to the situation before and after the bump, respectively, for the model of 0.9 M_⊙, [ Fe/H ] = −1.3. HBS: hydrogen burning shell; shaded zone: fingering unstable region; hatched area: innermost part of the convective envelope. The left and right panels are separated by 9.7 Myr of evolution.