Left panel: T_eff−log g diagram of RV variable helium-rich sdOB and sdO stars (see Fig. 1). The size of the symbols scales with ΔRV_max. The helium main sequence (HeMS) and the HB band are superimposed with HB evolutionary tracks (dashed lines) for subsolar metallicity (log z = −1.48) from Dorman et al. (1993). The three tracks correspond to helium core masses of 0.488, 0.490 and 0.495 M_⊙ (from bottom-left to top-right). Right panel: highest RV shift between individual spectra plotted against time difference between the corresponding observing epochs for helium-rich sdO and sdOB stars (see Fig. 2).