Comparison of evolutionary tracks of Althaus et al. (2013) and this work (including diffusion only) for a ~ 0.18 M_⊙ WD (left panel) and ~ 0.27 M_⊙ WD (right panel). The post-RLO evolution of luminosity, given by CNO burning, is plotted as a function of the hydrogen envelope mass. Time goes from right to left. The inset shows an artefact of hydrogen production during the shell flashes in the Althaus et al. (2013) models, see text.