T_eff–log g diagram showing the location of ELMV stars (light green triangles), pre-ELMV stars (blue diamonds), δ Sct stars (white circles) including SX Phe stars (magenta circles). The atmospheric parameters are extracted from different works detailed in the main text. Also, we included the theoretical evolutionary tracks of low mass He-core WDs (black dotted and dashed lines) from Althaus et al. (2013) and Serenelli et al. (2001), and of MS evolutionary tracks (red lines) from Sánchez Arias et al. (2017). Black numbers correspond to different values of the stellar mass of the low mass Hecore WDs, whereas red numbers are associated to the values of the mass, the metallicity and the overshooting parameter (see Table 3). Orange triangles show the position of the template models (see Sect. 4). The cyan squares indicate the position of the two stars reported by Corti et al. (2016), J1730+0706 and J1458+0707, and the cyan diamond represents the position of a new pulsating star, J0757+1448. These three objects will be analyzed in Sect. 6.