r(AB) magnitude vs. hard (2–10 keV) X-ray flux for the 250 X-ray emitting variable AGN candidates (large red symbols denote Chandra detections, and an open box denotes XMM detection). The small gray symbols denote the remainder of the X-ray sample. Leftward arrows indicate that only upper limits of the X-ray flux values are available. The dashed line corresponds to X/O = 0. The lower and upper solid lines represent X/O = −1 and X/O = 1, respectively, and define the AGN locus.