X-Ray Nova XTE J1550−564: Optical Observations

We report the identification of the optical counterpart of the X-ray transient XTE J1550-564 described in two companion papers by Sobczak et al. and Remillard et al. We find that the optical source brightened by ≈4 mag over the quiescent counterpart seen at B ≈22 on a SERC survey plate and then decayed by ≈1.5 mag over the 7 week-long observation period. There was an optical response to the large X-ray flare described by Sobczak et al., but it was much smaller and delayed by ≈1 day.