Upper left: T_eff − log g diagram of the hot subdwarf stars in the 500 pc sample, colour-coded as shown in the legend (lower panel). Three post-EHB evolutionary tracks for three different envelope masses from Han et al. (2003) are given as solid-black lines: 0.00, 0.001, and 0.005 M_⊙ from bottom to top (M_total = 0.45 M_⊙). The thickness of the lines are linearly proportional to the predicted evolutionary time. A double He-WD merger track from Zhang & Jeffery (2012) is shown as a orange dot-dashed line. The region encapsulated by the zero-age EHB and the terminal-age EHB from Dorman et al. (1993) is depicted as a shaded light-blue region which extends down to the He-MS from Paczyński (1971) which is shown as a dotted-grey line. All tracks are given for solar metallicity. Upper right: helium abundance – T_eff diagram for the same stars. The helium sequences identified in earlier studies (Edelmann et al. 2003; Németh et al. 2012; Luo et al. 2016; Lei et al. 2018) are shown. Bottom: Hertzsprung-Russell diagram with the same hot subdwarf classes and evolutionary tracks as shown in the upper left panel. The He-MS from Paczyński (1971) is again shown as a dotted-grey line with the corresponding masses annotated. The horizontal dashed-black lines indicate the solar metallicity ZAEHB at log L/L_⊙ = 1.05 at the start of the 0.45 M_⊙ evolutionary tracks from Han et al. (2002), and the TAEHB of these tracks at the point of core helium depletion. Two additional ZAEHB positions are marked as dashed-red lines for the 0.40 M_⊙ and 0.35 M_⊙ evolutionary tracks to compare with the low-luminosity objects. Black pentagons mark the underluminous hot subdwarfs (log L/L_⊙ < 1.05) in all three plots. Crimson pentagons indicate the three identified hot and helium-poor stars. The grey star marks the previous position of HD 188112.