Grid of low-mass He-core WD evolutionary sequences considered in this work shown in the M_⋆ − log(M_H/M_⋆) plane. The small circles represent each sequence of WD models with a given stellar mass and a specific thickness of the H envelope at T_eff ∼ 8000 K. The black circles connected by a thick (gray) line correspond to the values of the maximum H envelope thickness as predicted by the evolutionary computations of Althaus et al. (2013). For each sequence, we have pulsationally analyzed about 200 stellar models covering the interval of T_eff between 6000 and 10 000 K.