Colour – absolute magnitude diagram for members, O-type stars, and field stars in the search field centred on NGC 6618 (PG). The members and O stars are coloured and marked similarly as in Fig. 2. Absolute magnitudes are determined from individual parallaxes. Shown are three PARSEC isochrones with A_V = 2.0 mag and age of 0.5, 1.0 and 2.0 Myr with the solid, dashed and dot-dashed tracks, respectively. We give the reddening line (R_V = 3.1) for an O9–9.5 V star with the dashed black line and superimpose on this line the position of 2–14 mag of A_V, in steps of 2 mag, with the red crosses.