Solid curve: r_min for dust grains with κ_rp = 6.7 × 10³ cm² g^-1 plotted as a function of luminosity L, assuming typical parameters for the stellar velocity (=10 km^-1) and an ISM density similar to the WNM (=0.5 cm^-3). Overplotted are Galactic O-stars (open squares) and B-supergiants (open triangles) where wind parameters are taken from Puls et al. (1996) and Crowther et al. (2006). The filled circles are weak-wind objects from Bouret et al. (2003), Martins et al. (2004) and Najarro et al. (2011). σ Ori AB is marked as the filled square symbol.