T_eff − log g plane showing the low-mass He-core WD evolutionary tracks of Althaus et al. (2013; thin black lines). Sequences with H flashes during the early-cooling phase are depicted with dashed lines, sequences without H flashes are displayed with solid lines. Numbers correspond to the stellar mass of each sequence. The locations of the five known pulsating low-mass WDs (Hermes et al. 2013a) are marked with a small circle (red). Stars not observed to vary are depicted with green triangles. Black circles and squares on the evolutionary tracks of M_∗ = 0.1554 M_⊙ and M_∗ = 0.2389 M_⊙ indicate the location of the template models analysed in Sect. 3.2.