Comparison between the VST COSMOS catalog (smaller red dots) and the Chandra (left panel) and XMM (right panel) COSMOS catalogs (larger blue dots). The holes in the VST catalog represent regions that were masked and hence not taken into account in our variability analysis because of the presence of bright star halos (see Sect. 3). It is apparent that part of the COSMOS field as imaged by the VST is not covered by Chandra observations. In both panels we showed all the sources in each of the X-ray catalogs, but we limited our analysis to those falling in the VST FoV, out of the masked areas.