T_eff−log g plane showing the low-mass He-core WD evolutionary tracks (final cooling branches) of Althaus et al. (2013). Numbers correspond to the stellar mass of each sequence. The locations of the seven known ELMVs (Hermes et al. 2012, 2013b,a; Kilic et al. 2015; Bell et al. 2015) are marked with large red circles (T_eff and log g computed with 1D model atmospheres) and small black circles (after 3D corrections). Stars observed not to vary (Steinfadt et al. 2012; Hermes et al. 2012, 2013b,a) are depicted with hollow small blue circles. The hollow square on the evolutionary track of M_⋆ = 0.1762 M_⊙ indicates the location of the template model analyzed in Sect. 3. The gray-shaded region bounded by the dashed blue line corresponds to the instability domain of ℓ = 1g modes according to nonadiabatic computations using ML2 (α = 1.0) version of the MLT theory of convection; see Sect. 3.2.