We present simultaneous X-ray ( RXTE ) and optical ( ULTRACAM ) narrow band ( Bowen blend/HeII and nearby continuum ) observations of Sco X-1 at 2-10 Hz time resolution .
We find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of \sim 11 - 16 s which is consistent with reprocessing in the companion star .
The echo from the donor is detected at orbital phase \sim 0.5 when Sco X-1 is at the top of the Flaring Branch .
Evidence of echoes is also seen at the bottom of the Flaring Branch 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 .