Distribution in the sHRD of the birth location of mass gainers as predicted by the binary evolution computations by Jin et al. (2026). The full sample of gainers are depicted in grey, while those with enriched He abundances are highlighted with colours separating the mass-transfer cases A (red), B (orange), and AB (blue). Single star evolutionary tracks computed with the MESA stellar evolution code by Jin et al. (2024a) and used as the basis for the binary evolution computation are also depicted for reference purposes. The open circles show the location of the He-rich group of O-type stars from our study.