Coordinated X-ray and optical observations of Scorpins X-1.

The authors present the results of coordinated, partly simultaneous, optical and X-ray (Ginga) observations of the low-mass X-ray binary Sco X-1. They find that the division between the optically bright and faint state, at a blue magnitude B = 12.8, corresponds to the change from the normal to the flaring branch in the X-ray colour-colour diagram as proposed by Priedhorsky et al. (1986). From archival Walraven data it is found that in both optical states the orbital light curve is approximately sinusoidal, and has similar amplitudes.