The optical light curve of the low-mass X-ray binary XB 1254-690

The discovery of periodic optical variations from the counterpart of the low-mass X-ray binary XB 1254-690 is reported. The period is 0.163890 + or - 0.000009 days, identical to the recurrence period of X-ray dips. The full amplitude of the light curve is 0.4 mag and the mean V-magnitude is 19.1. From spectrophotometric observations the authors derive a B-V color index of 0.31 + or - 0.05 and an interstellar reddening of E(B-V) = 0.4 + or - 0.1. The optical minimum occurs 0.15 in phase after X-ray dips. Most of the optical light curve is probably due to varying aspect of the X-ray-heated secondary atmosphere. The contribution of the X-ray-heated bulge does not dominate the optical variability. The X-ray-heated secondary explanation supports the interpretation that the X-ray dips are due to eclipses by a bulge located on the edge of the accretion disk and indicates that at least in the case of XB 1254-690 the accretion disk does not shadow completely the companion star.