The soft X-ray transient GS2000 + 25-I. An 8.3-hr modulation during outburst

Optical spectroscopy of the X-ray transient, GS2000 + 25, during its 1988 outburst shows that this system is similar to A0620 - 00, but at a greater distance (> 2 kpc). We present photometry from La Palma and Russia that reveal a ∼ 0.1 -mag modulation at a period of 8.33 hr which we associated with the orbital period. We propose that the modulation, which is in the form of a 'hump' in the light curve, is analogous to the mechanism of an eccentric, precessing accretion disc used by Whitehurst (1988) to account for superhumps in SU UMa systems. If confirmed, this would strengthen the relationship between the outburst behaviour of low mass X-ray binaries and cataclysmic variables.