  We have discovered a persistent, but highly variable X-ray source in the
nearby starburst galaxy NGC 253. The source varies at the level of a factor of
about 5 in count rate on timescales of a few hours. Two long observations of
the source with Chandra and XMM-Newton show suggestive evidence for the source
having a period of about 14-15 hours, but the time sampling in existing data is
insufficient to allow a firm determination that the source is periodic. Given
the amplitude of variation and the location in a nuclear starburst, the source
is likely to be a Wolf-Rayet X-ray binary, with the tentative period being the
orbital period of the system. In light of the fact that we have demonstrated
that careful examination of the variability of moderately bright X-ray sources
in nearby galaxies can turn up candidate Wolf-Rayet X-ray binaries, we discuss
the implications of Wolf-Rayet X-ray binaries for predictions of the
gravitational wave source event rate, and, potentially, interpretations of the
events.
