IS GS2000+25 AN ECLIPSING BLACK-HOLE BINARY

We present an alternative interpretation of the quiescent optical ligh t curve of GS2000 + 25 published by Chevalier & Ilovaisky, and show th at the modulation can be explained by a single periodicity. Using a me thod that is independent of the characteristics of the modulation of t he 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 & Ilovaisk y's orbital period. The modulation can either be explained as primaril y due to the ellipsoidal variations of the secondary star or as a comb ination of the above and an eclipse. From X-ray data taken during outb urst and decline, we find no evidence of an eclipse of the compact obj ect, and therefore the eclipse required must involve the secondary sta r and the accretion disc. Assuming an accretion disc comparable to tha t in A0620 - 00, for the eclipsing case we find the system to lie in t he inclination range 62-degrees-82-degrees, and for the ellipsoidal ca se in the range 56-degrees-79-degrees.