A 106 day period in the nuclear source X-8 in M33

With an X-ray luminosity of about 10(39) ergs s(-1), the source X-8, coincident with the optical center of M33, is the most luminous X-ray source in the Local Group. However its nature remains a mystery. We present here new and archival ROSAT observations of X-8 spread over 6 yr that show variability and a periodicity of similar to 106 days. This implies that (most of) the emission from M33 X-X arises from a single object, perhaps a binary system with a black hole primary of similar to 10 M.. We suggest that the companion is a giant orbiting with a period of similar to 10 days and that the observed modulation is ''superorbital,'' analogous to that seen in Cyg X-2 and X1820-30.