  We report the discovery of an eclipsing X-ray binary in M101, the first such
system to be discovered outside the Local Group. Based on a sequence of 25
Chandra observations that sample a wide range of orbital phases, we find a
period of 32.688 $\pm$ 0.002 hours, which we interpret as an orbital period.
The folded light curve exhibits an eclipse lasting about 8 hours, suggesting a
compact orbit in an nearly edge-on configuration. The X-ray binary has an
average luminosity of $L_X(0.3-8 {\rm keV}) \approx 1.3\times10^{38}$ erg/sec,
with only one out of the 25 observations significantly lower in flux than the
average light curve.The presence of the eclipse and the $\sim$ 1.4-day orbital
period suggests that this source is an analog of the well studied eclipsing
X-ray binary pulsars Her X-1 or LMC X-4. Combining the Chandra data and the HST
ACS/WFC images, we have identified several possible optical counterparts,
including an O5-O3 star with V = 25.0. Follow-up optical monitoring
observations should be able to identify the donor and further constrain the
orbital properties.
