Upper panel: Kiel diagram (T_eff−log g). He-poor stars are indicated with blue circles, iHe subdwarfs with black circles, and eHe subdwarfs with red circles. The zero-age and terminal-age EHBs for a subsolar metallicity of −1.48 (solid grey lines) were interpolated from evolutionary tracks by Dorman et al. (1993). The helium main sequence (dashed line) is taken from Paczyński (1971). The evolution of a merging helium white dwarf (He-WD) system (Yu et al. 2021) is shown as a light blue line. Lower panel: helium abundance as a function of T_eff. The lower and upper limits to log [n(He)/n(H)] are marked by upward and downward arrows. The iHe-rich subdwarfs are distinguished from the He-poor subdwarfs by the solid line, and from eHe-rich subdwarfs by the dashed line.