T_eff − log g-diagram taken from Wu et al. (2018) with BPS BS 16981−0016 marked a yellow diamond. Evolutionary tracks for donor stars with different initial masses on the MS (colour-coded) are plotted during the He-core burning phase along with atmospheric parameters of a sample of known hot subdwarfs taken from Geier et al. (2017a). The solid lines show tracks for He cores, which were degenerate before their ignition (M_d ≤ 2.0 M_⊙), and the dashed lines show those from non-degenerate He cores (M_d ≥ 2.5 M_⊙).