RV variability and helium abundance. Upper panel: T_eff − log n(He)/n(H) diagram of the full sample. Helium-poor stars are marked in grey, intermediate helium-rich ones in orange and extreme helium-rich ones in red. The size of the symbols scales with ΔRV_max. Solar helium abundance is marked by the orange horizontal line, while the red line marks the transition between intermediate and extreme helium abundance. Middle panel: same diagram for the stars showing significant RV variability (log p < −4.0). Lower panel: log p distribution of the sample using the same colour-coding as in the upper panels. The significance level log p < −4.0 is marked by a vertical line. The distribution has been limited to log p > −10 and all objects with smaller log p have been stacked in the first bin for visualisation.