Colour−magnitude diagram of stars in the region of the central cluster, with the two central O-type stars indicated by the large blue symbols. PMS isochrones from Siess et al. (2000, for Z = and ages of 0.5, 1, 2, and 3 Myr (from right to left), corrected for a distance of 50 kpc and average extinction of A_V = 2.8 mag, are indicated by the solid black lines, with the zero-age main sequence from Marigo et al. (2008) shown by the blue line.