NIR colour-magnitude diagram for our targets in M17. The objects for which we confirm their mYSO nature are denoted by blue circles, the stars with NIR excess only longward of 2.5 μm, but without emission lines, are indicated with green triangles, and the O and B main-sequence stars are indicated with grey squares. The red star corresponds to B358, which is most probably a background post-AGB star. The location of the zero-age main sequence is shown for a distance of 1.98 kpc with a solid black line (H97; Cox 2000). In grey we overplot the updated parameters of O V stars according to Martins & Plez (2006). The dashed line represents the reddening line for an O3 V star with R_V = 3.1. Some of the mYSO candidates are located above the reddening line indicating the presence of a NIR excess.