XMM Lockman Hole sources plotted on the Stern et al. (2005) diagram. In blue and red circles are plotted the soft and hard X-ray sources according to their hardness-ratios, while large symbols mark sources with bright optical and mid-infrared fluxes (R_AB < 21.5 and f_3.6 μm > 12 μJy) to imitate the selection criteria used by Stern et al. (2005). Grey dots represent the mid-infrared colours of all IRAC detections in the Lockman Hole, and the lines are tracks of different SED templates with 0 < z < 4. This figure is available in color in electronic form.