Evolutionary tracks (in color lines) for samples of O dwarfs, giants, and supergiants. For each model, the central H exhaustion is indicated by crosses. The zero-age line is in dashed black. Stars are grouped by different symbols and colors. Our sample (red crosses) has initial masses (M_ZAMS) ranging around 25–28 M_⊙. Late O giants are closer to the end of the main sequence phase than the dwarfs.