Mean molecular weight profile as a function of the fractional mass in the interior of 1.25 M_⊙ models at three different evolutionary stages (cf. Fig. 1): early post-TAMS (red, diamonds), subgiant (green, squares) and red giant (blue, circles). The approximate formula , valid for a fully ionized gas, has been used. Ages in Gyr and log g are shown on the side. The filled and open symbols on each curve mark the He core outer boundary and the bottom of the outer convection zone, respectively. Dotted lines indicate the pure helium μ = 4 / 3 and pristine composition μ ≈ 0.6.