  We discovered that the Wolf-Rayet (WR)+OB star binary, WR 104, renowned for
its associated "dusty pinwheel nebula" recently spatially resolved with
infrared interferometry, exhibits strong quasi-periodic optical variations with
a full amplitude of 2.7 mag. Such a large-amplitude, continuous variation has
been unprecedented in a WR star. The optical quasi-period (~241 d) is in almost
perfect agreement with the interferometric period (243.5+/-3 d). The remarkable
agreement of the dominant period in optical variability with the orbital period
supports that the strongly varying dust obscuration is physically related to
the binary motion, rather than sporadic dust-forming episodes. Considering the
low orbital inclination (11+/-7 deg) and the nearly circular orbit inferred
from the interferometric observations, the strongly variable line-of-sight
extinction suggests that the highly structured extinction can be being formed
via an ejection of dust in the direction of the binary rotation axis. Another
viable explanation is that the three-dimensional structure of the shock front,
itself is the obscuring body. Depending on the geometry, the dusty shock front
near the conjunction phase of the binary can completely obscure the inner
WR-star wind and the OB star, which can explain the amplitude of optical fading
and the past observation of remarkable spectral variation.
