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We report on the first multi-color precision light curve of the brightWolf-Rayet binary $\gamma^2$ Velorum, obtained over six months with thenanosatellites in the BRITE- Constellation fleet. In parallel, we obtained 488high-resolution optical spectra of the system. In this first report on thedatasets, we revise the spectroscopic orbit and report on the bulk propertiesof the colliding winds. We find a dependence of both the light curve and excessemission properties that scales with the inverse of the binary separation. Whenanalyzing the spectroscopic properties in combination with the photometry, wefind that the phase dependence is caused only by excess emission in the lines,and not from a changing continuum. We also detect a narrow, high-velocityabsorption component from the He I $\lambda$5876 transition, which appearstwice in the orbit. We calculate smoothed-particle hydrodynamical simulationsof the colliding winds and can accurately associate the absorption from He I tothe leading and trailing arms of the wind shock cone passing tangentiallythrough our line of sight. The simulations also explain the general strengthand kinematics of the emission excess observed in wind lines such as C III$\lambda$5696 of the system. These results represent the first in a series ofinvestigations into the winds and properties of $\gamma^2$ Velorum throughmulti-technique and multi-wavelength observational campaigns.

The variability of the BRITE-est Wolf-Rayet binary, $\gamma^2$ Velorum I. Photometric and spectroscopic evidence for colliding winds