The Orbital Period and Time-variable Asymmetric Accretion Disk in the X-Ray Binary MS 1603.6+2600 (=UW Coronae Borealis)
We present CCD photometry of the low-mass X-ray binary UW Coronae Borealis (UW CrB). Its light curve shows eclipses at a period near 111 minutes, but the eclipses vary in depth and shape and often disappear. Restricting our analysis to the deeper eclipses, we find the orbital period to be 110.976722 +/- 0.000012 minutes, but the times of mideclipse can deviate by more than 0.025 in phase from the best-fit ephemeris. There is an additional large-amplitude variation with a period of 112.58 +/- 0.03 minutes reminiscent of the superhumps seen in the light curves of some cataclysmic variables. The variations of the eclipse morphology are not random, repeating at a period near 5.5 days, and the shape of the superhump-like modulation also varies at this period. We interpret the light curve as the eclipse of the accretion disk around the neutron star by the secondary star. The surface brightness of the accretion disk is strongly asymmetric and highly variable, producing the variations of the eclipse morphology and times of mideclipse. A model in which the distribution of surface brightness is elliptical and precesses at the 5.5 day period reproduces the eclipse depths and the times of mideclipse reasonably well. As 112.6 minutes is the beat period between 110.97672 minutes and 5.5 days, the superhump-like variability is closely related to the precessing elliptical disk, but the causal relationship is unclear.