Optical (R_C-band) versus X-ray (0.5 − 10 keV) flux of the XMM-Newton sources. With blue and red symbols we plot the soft and hard X-ray sources respectively, which are characterised by their hardness-ratio, where HR =  −0.4 is the dividing value. Different shapes are used according to the positions in the IRAC colour–colour wedge, similar to Figs. 4 and 5. Crosses mark the positions of EROs (with R_C − K > 5) and lines mark log (f_x/f_o) = 1,  − 1, and  − 2. This figure is available in color in electronic form.