Individual abundances of carbon (left), nitrogen (middle), and oxygen (right) as a function of surface gravity. The stars in the present sample are indicated with filled blue circles, while stars taken from our previous studies are shown with open blue circles. The surface gravity (obtained from the binary solution) is used as a proxy for stellar evolution. Solid red lines show theoretical evolutionary tracks for a 15 M_⊙ star and three values of the initial rotational velocity, Ω/Ω_crit = 0.1, 0.3, and 0.5 (Georgy et al. 2013). The cosmic standard abundance values of Nieva & Przybilla (2012) are indicated with horizontal dashed lines.