Top panel: M(log T_eff,log g) relation as obtained directly from stellar evolution simulations. Middle panel: M(log T_eff,log g) relation derived from the weighted least square scheme described in Sect. 4. Bottom panel: uncertainty σ_M of the derived M(log T_eff,log g) relation. Symbols correspond to the observed post-RGB low-mass stars listed in Table 2. In particular, green circles correspond to the pulsating ELM white dwarfs detected by Hermes et al. (2013). White regions in the middle and bottom panels indicate either fast evolutionary stages for which not enough points were available to obtain an accurate fit (in the case of CNO-flashing sequences) or simply that no tracks are available at that region.