We present simultaneous X-ray (RXTE) and optical (ULTRACAM) narrow-band (Bowen blend/He ii and nearby continuum) observations of Sco X-1 at 2-10 Hz time resolution. We find that the Bowen/He ii emission lags the X-ray light curves with a light traveltime of s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ∼0.5 when Sco X-1 is at the top of the flaring branch (FB). Evidence of echoes is also seen at the bottom of the FB but with time-lags of 5-10 s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.
