Colour–magnitude diagram for stars projected 2′ around the centre of the superbubble. Plotted are ~1900 stars from the photometric catalogue of Zaritsky et al. (2004). The dashed lines show the evolutionary tracks for stars with 1, 3 (light blue), 5 (dark blue), 7 and 12 M_⊙ (marked in red if not stated differently, from bottom to top). The two most massive stars are marked with yellow star symbols. Two isochrones at 28 Myr (green) and 35 Myr (purple), which are taken from Lejeune & Schaerer (2001), are plotted as solid curves.