Fractional abundance of H (X_H) versus the outer mass fraction coordinate, for ELM WD models with M_⋆ = 0.1554 M_⊙ and T_eff = 8000 K. The lowest curve (thick black) corresponds to a model characterized by the thickest (canonical) envelope for this stellar mass, while the upper curves (which have been artificially displaced upwards for clarity) correspond to models with H envelopes having decreasing thicknesses. The values of M_H are shown at the right of the figure. Black (blue) curves correspond to the case of H envelopes with single-layered (double-layered) chemical structures. Black dots indicate the location of each step in the chemical profile.