Fraction of the total white dwarf luminosity due to hydrogen nuclear burning (CNO and pp burning) for all our helium-enhanced sequences for metallicity Z = 0.0005 and Z = 0.001 (upper and bottom panel, respectively) and initial helium mass fraction Y = 0.4. Red (blue) lines show the predictions of our sequences that end as white dwarfs with helium (C/O) cores. The green line corresponds to a helium-core white dwarf sequence of 0.435 M_⊙ resulting from a progenitor with Z = 0.01 and standard initial helium abundance, taken from Althaus et al. (2013). For less massive C/O white dwarfs, residual hydrogen burning becomes a relevant energy source even at low luminosities.