Panel A) distribution of stars as a function of binary probability. There are more Y stars with high probabilities of being binaries than O stars. Panel B) [C/N] abundances as a function of mass for our sample of stars. The vertical dashed line marks the mass threshold between O and Y stars. Red symbols denote stars with probabilities larger than 68% of being binaries, while black symbols denote stars more likely to be single. The blue solid line indicates the theoretical relation between [¹²C/¹⁴N] and stellar mass in a post-FDU population, from Lagarde et al. (2012). There is no significant relation between [C/N] and binarity in the sample and several Y stars do not follow the theoretical relation. Panel C) [C/N] abundances as a function of surface gravity. Blue circles correspond to the Y stars and pink triangles represent O stars.