X-ray-to-optical flux ratio versus R_C − K colour of the X-ray AGN in the Lockman Hole. Blue and red colours represent hard and soft X-ray sources respectively according to their hardness-ratio, and different symbols represent the position of the IRAC counterpart in the Stern diagram (Fig. 3). Filled circles are sources in the wedge (with MIR colours typical of AGN), open circles are sources outside the wedge (with MIR colours typical of non-active galaxies) and asterisks are sources without full IRAC photometry, either because they are fainter than the IRAC limiting magnitude or because they are out of the field. The solid line is the best-fit line to the positions of the filled circles, while the dashed lines mark its 1σ limits. This figure is available in color in electronic form.