Helium abundance as a function of projected rotational velocity for the O Standard stars sample. Upper and lower limits are represented by arrows. Different colors and shapes represent luminosity classes. Open symbols indicate stars for which we have detected clear or likely signatures of spectroscopic binarity. Dashed gray lines delimit the range of values in v sin i and Y _He in which most of the stars are concentrated: 50–100 km s⁻¹ and 0.08–0.12, respectively.