Position of the O-type stars of NGC 6231 in a Hertzsprung-Russell diagram, accounting for the binarity. The components of HD 152218 are shown by the filled magenta symbols. Evolutionary tracks (identified with their initial mass) for Z = 0.014 are taken from Ekström et al. (2012). The red lines yield isochrones for log age = 6.5 and 6.6 (dashed lines, i.e. ages of 3.2 and 4.0 Myr, from left to right), and log age = 6.71,6.76 and 6.81 (continuous lines, i.e. ages of 5.1, 5.8 and 6.5 Myr, from left to right). The latter isochrones bracket the range of ages inferred from the apsidal motion of HD 152218. The leftmost panel corresponds to evolutionary models without rotation, whilst the rightmost panel yields the tracks and isochrones for stars rotating at 40% of their critical velocity.