SIMBAD references

There is increasing evidence that single-star evolutionary models are unable to reproduce all of the observational properties of massive stars. Binary interaction has emerged as a key factor in the evolution of a significant fraction of massive stars. In this study, we investigate the helium (Y_He) and nitrogen (ϵ_N) surface abundances in a comprehensive sample of 180 Galactic O-type stars with projected rotational velocities of ≤150 km s^–1. We found a subsample (∼20% of the total, and ∼80% of the stars with Y_He ≥ 0.12) with a Y_He and ϵ_N combined pattern that is unexplainable by single-star evolution. We argue that the stars with anomalous surface abundance patterns are binary interaction products.