Left panel: colour-colour diagram with the classes of observed anomalies marked as different symbols: broadline QSOs are shown as asterisks, BAL QSOs as diamonds, a YSO as a triangle, galaxies as crosses, a Seyfert galaxy as a square, and unidentified objects as plus signs. Right panel: W1 magnitude in Vega plotted against g_AB magnitude for the targeted sources (same colour and symbol coded as for left panel). Grey contours and points mark the position of all OCSVM anomalies contained in the S17 catalogue which have optical detection with g_AB <  19.5.