Left: smoothed CMD for field RX J1524 containing the CFHT sources classified as stars in our work. Right: same diagram for the sources classified as stars in the SDSS database. The weaker features in the left figure are not detectable in the right figure due to photometric uncertainties and the different star/galaxy separations. The smoothing two-dimensional kernel is the standard bivariate normal density; the function mapping the density scale to the colour scale follows x^0.25.