IS GS2000+25 AN ECLIPSING BLACK-HOLE BINARY

We present an alternative interpretation of the quiescent optical light curve of GS2000 + 25 published by Chevalier & Ilovaisky, and show that the modulation can be explained by a single periodicity. Using a method that is independent of the characteristics of the modulation of the light curve, we obtain an alternative value for the orbital period of 0.2935 d, which is twice the 1-d alias of half Chevalier & Ilovaisky's orbital period. The modulation can either be explained as primarily due to the ellipsoidal variations of the secondary star or as a combination of the above and an eclipse. From X-ray data taken during outburst and decline, we find no evidence of an eclipse of the compact object, and therefore the eclipse required must involve the secondary star and the accretion disc. Assuming an accretion disc comparable to that in A0620 - 00, for the eclipsing case we find the system to lie in the inclination range 62-degrees-82-degrees, and for the ellipsoidal case in the range 56-degrees-79-degrees.