Upper panel: fractional abundance by mass of selected nuclear species in terms of the outer mass fraction, corresponding to a DA WD model characterized by M_⋆ = 0.584 M_⊙, T_eff = 19 000 K and log(M_H/M_⋆) = − 13.3. Lower panel: same as in the upper panel, but with a logarithmic scale for the chemical abundances. In this panel, the black and red dashed lines correspond to initial fractional abundances of H and He, respectively.