Mass distribution histogram (filled blue bars) of YSOs. These YSOs were selected using our new photometric colour–colour criteria from the SIRIUS catalogue within | l | < 1.̊5 and | b | < 0.̊5. Masses are determined approximately using the YSO parameters from SED fits and pre-main sequence tracks of Bernasconi & Maeder (1996). The black curve represents the Kroupa IMF (Kroupa 2001) fitted to the peak of the distribution. We estimate the mass of the underlying YSO population from the area under the curve from 0.01 M_⊙ to 120 M_⊙.