  We observe the HMXB BP Cru using interferometry in the near-infrared K band
with VLTI/GRAVITY. Continuum visibilities are at most partially resolved,
consistent with the predicted size of the hypergiant. Differential visibility
amplitude ($\Delta |V| \sim 5\%$) and phase ($\Delta \phi \sim 2 \deg$)
signatures are observed across the HeI $2.059 \mu$m and Br$\gamma$ lines, the
latter seen strongly in emission, unusual for the donor star's spectral type.
For a baseline $B \sim 100$m, the differential phase RMS $\sim 0.2 \deg$
corresponds to an astrometric precision of $\sim 2 \mu$as. A model-independent
analysis in the marginally resolved limit of interferometry reveals asymmetric
and extended emission with a strong wavelength dependence. We propose geometric
models based on an extended and distorted wind and/or a high density gas
stream, which has long been predicted to be present in this system. The
observations show that optical interferometry is now able to resolve HMXBs at
the spatial scale at which accretion takes place, and therefore probe the
effects of the gravitational and radiation fields of the compact object on its
environment.
