Relative magnitude variation of CoRoT-23, as measured with CFHT-MEGACM, at two time intervals of the night 07 Sept. 2010, the first one (noted ON) during the predicted transit. The expected variation due to the transit is detected with a significant signal-to-noise ratio as shown by the 1σ range (dotted lines) for each sequence; it corresponds in amplitude and sign to the predicted one.