2009ApJ...697L..14H

We present optical and infrared photometry of the low-mass X-ray binary EXO 0748-676 in quiescence for the first time in 24 years since it became X-ray active in 1985. We find the counterpart at average magnitudes of R = 22.4 and J = 21.3. We monitored the source approximately nightly from 2008 November to 2009 January. During this time there was considerable night-to-night optical variability but no long-term trends were apparent. The night-to-night variability reveals a periodicity of P = 0.159331±0.000012 d, consistent with the X-ray orbital period to within 0.01%. This indicates that the quiescent optical modulation is indeed orbital in nature rather than a superhump. Interestingly, the modulation remains single-peaked with a deep minimum coincident with the times of X-ray eclipse, and there is no indication of a double-peaked ellipsoidal modulation. This indicates that even in "quiescence," emission from the accretion disk and/or X-ray heated inner face of the companion star dominate the optical emission, and implies that obtaining an accurate dynamical mass estimate in quiescence will be challenging.