T_eff − log g diagram of different subclasses of single-lined He-poor hot subdwarfs. The left panel shows our sample, and the right panel shows the sample of Geier et al. (2022). The circles represent non-RV-variable hot subdwarfs, and the crosses represent RV-variable hot subdwarfs. The sample is divided into six subclasses that are marked by different colors, for which EHBa, EHBb, EHBc, bEHB, postEHBa, and postEHBb are plotted in blue, red, green, purple, black, and magenta, respectively. The zero-age EHB (ZAEHB) and terminal-age EHB (TAEHB) sequences with Z = 0.02 from Dorman et al. (1993) are labeled by solid black lines. The helium main sequence (He-MS) from Paczyński (1971) is marked by the dashed black line.