Impact of residual nuclear burning on the cooling times of three selected white dwarf sequences resulting from progenitors with Z = 0.0001. The solid line illustrates the evolution in the case in which H burning is taken into account, whereas the dotted line shows the case in which nuclear burning is disregarded. For the 0.53512 M_⊙ white dwarf cooling sequence we also show the evolution when diffusion is neglected (dashed line).