Diagrams showing the selection of the PMS stars. Panel a): CCD showing the V − I and V − Hα colours of the stars. The dashed red line indicates the theoretical colour, following the Bessell et al. (1998) models. Black dots indicate the stars with Hα excess larger than 5σ and an EW smaller than −20 Å. Red dots indicate stars with Hα excess larger than 5σ but which have an EW larger than −20 Å. Following our EW selection criterion, these stars are thus not included in our sample of bona fide PMS stars. Panel b): EW in Hα plotted against the V − I colour of the stars. The black dots indicate the stars with Hα excess larger than 5σ and an EW smaller than −20 Å, and the red dotted line indicates an EW of −20 Å.