Estimating the low-mass IMF in OB associations : o Orionis

We investigate the issues of contamination and exclusion in photometric membership selection techniques when applied to OB associations. We use radial velocities to calculate membership probabilities for a sample of very low-mass candidate members of the σ Orionis young group. We find that significant numbers of bona members are not excluded by photometric selection techniques, and for I brighter than 17 there is not significant contamination in the expected PMS region.