Time variation of the mass at the bottom of the convective envelope (black, scale on the left), of the surface sodium (blue, scale on the left), and of the carbon isotopic ratio (red, scale on the right) of a 2.6 M_⊙ model star calculated with an overshoot parameter of ζ = 0.04. The plot is focussed on the epoch during which the FDU takes place. The dashed blue line indicates the variation of the surface sodium abundances of a model star, calculated by assuming that the initial neon is enhanced by a factor of three.