  With an X-ray luminosity of about 10^39 erg/s, 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 years which show variability and a ~ 106-d
periodicity. This implies that (most of) the emission from M33 X-8 arises from
a single object, perhaps a binary system with a ~ 10 Msol black hole primary.
We suggest that the companion is a giant orbiting with a ~ 10 day period, and
that the observed modulation is ``super-orbital'', analogous to that seen in
Cyg X-2 and X1820-30.
