  We present the results of our study of the luminous (L_{X} ~ 10^{39} erg/s)
X-ray binary CXOU J121538.2+361921 in NGC 4214, the high mass X-ray binary with
the shortest known orbital period. Using Chandra data, we confirm the ~13,000 s
(3.6 hr) eclipse period, and an eclipse duration of ~2000 s. From this, we
estimate a mass ratio M_2/M_1 >~ 3 and a stellar density of about 6 g cm^{-3},
which implies that the donor must be a Wolf-Rayet or a stripped Helium star.
The eclipse egress is consistently much slower than the ingress. This can be
explained by denser gas located either in front of the compact object (as
expected for a bow shock) or trailing the donor star (as expected for a shadow
wind, launched from the shaded side of the donor). There is no change in X-ray
spectral shape with changing flux during the egress, which suggests either
variable partial covering of the X-ray source by opaque clumps or, more likely,
a grey opacity dominated by electron scattering in a highly ionized medium. We
identify the optical counterpart from Hubble images. Photometry blueward of
~5500 Ang indicates a bright (M_{B} = -3.6 +/- 0.3 mag, for a range of
plausible extinctions), hot (T = 90,000 +/- 30,000 K) emitter, consistent with
the Wolf-Rayet scenario. There is also a bright (M_{I} ~ -5.2 mag), cool (T =
2700 +/- 300 K) component consistent with an irradiated circumbinary disk or
with a chance projection of an unrelated asymptotic giant branch star along the
same line of sight.
