Identification of objects with excess Hα emission. The grey dots refer to the 6008 stars with good photometry in all bands. The blue dashed line is the photospheric V − Hα colour for the model atmospheres of Bessell et al. (1998) for the metallicity appropriate to the SMC, along which normal stars are expected to be found. The dashed lines of constant equivalent width corresponding to −20 Å, −50 Å, −100 Å, −200 Å, and −400 Å are also shown. Red dots denote stars with Hα excess emission higher than the 5σ level and W_eq < −20 Å. The error bars show the uncertainties in the two colours, which are sometimes smaller than the size of the symbols.