H − K vs. K colour–magnitude diagram of W49. All sources identified in JHK, with errors less than 0.1 mag and with K ≤ 16 mag plotted in grey. The blue diamonds represent the spectroscopically identified OB stars, red triangles are the YSOs with CO emission in their spectra, while the green squares indicate the location of the stars with Brγ absorption lines as the most obvious feature, which cannot be classified due to a lack of other features (see discussion in the text). The purple circles are the 22 late type stars idenfied as fore- or background stars. The dashed vertical line represents the unreddened isochrone for main sequence stars with an age of 1 Myr (Ekström et al. 2012; Yusof et al. 2013). The black star symbols on the dashed vertical line mark the O stars with subtypes O3V to O5V with a step of 1, and O5V to O9.5V in steps of 0.5. The black stars in the bottom right mark the locations of O stars of different subtypes for A_Ks = 5 mag. The two diagonal dashed lines indicate the reddening directions of an O3V and O9.5V star according to the extinction law of Indebetouw et al. (2005).