Soft (0.5–2 keV, left panel) and hard (2–10 keV, right panel) X-ray vs. optical flux for the VST variable objects with an X-ray counterpart from Chandra (dots) or XMM (squares) catalogs; Chandra fluxes were used when available, while XMM fluxes were adopted for the remaining objects. Downward arrows stand for those sources for which only upper limits of the X-ray flux were available. The smaller grey symbols in the background are shown as a reference population and stand for the X-ray objects in the Chandra catalog having a VST counterpart and r(AB) < 23 mag. The dashed line corresponds to X/O = 0, the lower and upper solid lines represent X/O = −1andX / O = 1, respectively. AGNs typically place themselves in the range where −1 ≤ X/O ≤ 1.