Color–magnitude diagram of stars selected using the criteria given by Eqs. (1) and (2). Stars retrieved by those criteria but with confirmed later spectral types (see Sect. 3.1) have been excluded. Filled circles are O and B-type stars spectroscopically confirmed, either by previous studies or by the present one, and dots are O and B-type candidates lacking spectroscopic confirmation. The solid line marks the position of the unreddened upper main sequence based on the “observational” T_eff scale of Martins et al. (2006) shifted to a distance modulus DM = 10.8. The dotted line represents the displacement caused in the diagram by an extinction of A_V = 5 mag, using the reddening law of Cardelli et al. (1989) with total-to-selective extinction ratio R_V = 3.1. The sharp cuts in the distribution of points are due to our magnitude limits K_S < 9, B < 16.