Left panel: colour–colour diagram for previously known arcs in red and a sub-sample of galaxies observed in the COSMOS field subdivided in redshift bins. The red points denote the arcs previously known in the survey that were used to calibrate the colour selection. The small colour region with arcs shows were the lensed sources lie at redshift z ≈ 1 (as predicted by our optical depth model) and have well-defined colour properties (as shown by the black arrow referring to a Scd galaxy, for which we derived the photometry out of a synthetic SED; here the arrowhead points in the direction of increasing redshifts). Right panel: same colour–colour diagram, but for all objects detected by the arcfinder.