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Abstract Massive-star binaries are critical laboratories for measuring masses and stellar wind mass-loss rates. A major challenge is inferring viewing inclination and extracting information about the colliding-wind interaction (CWI) region. Polarimetric variability from electron scattering in the highly ionized winds provides important diagnostic information about system geometry. We combine for the first time the well-known generalized treatment of Brown et al. for variable polarization from binaries with the semianalytic solution for the geometry and surface density CWI shock interface between the winds based on Cantó et al. Our calculations include some simplifications in the form of inverse-square law wind densities and the assumption of axisymmetry, but in so doing they arrive at several robust conclusions. One is that when the winds are nearly equal (e.g., O+O binaries) the polarization has a relatively mild decline with binary separation. Another is that despite Thomson scattering being a gray opacity, the continuum polarization can show chromatic effects at ultraviolet wavelengths but will be mostly constant at longer wavelengths. Finally, when one wind dominates the other, as, for example, in WR+OB binaries, the polarization is expected to be larger at wavelengths where the OB component is more luminous and generally smaller at wavelengths where the WR component is more luminous. This behavior arises because, from the perspective of the WR star, the distortion of the scattering envelope from spherical is a minor perturbation situated far from the WR star. By contrast, the polarization contribution from the OB star is dominated by the geometry of the CWI shock. 

Massive Wolf-Rayet (WR) stars in binary systems may produce supernovae capable of emitting long-duration gamma-ray bursts (LGRB). The canonical WR+O eclipsing binary is V444 Cygni, which is a WN5+O system that has X-ray emitting colliding winds and a well-constrained geometry. I will present new time-dependent spectropolarimetric data, collected using RSS at the Southern African Large Telescope, from several southern WN+O binary systems that may be analogs to V444 Cygni. By analyzing their polarimetric variations with respect to V444 Cygni, I investigate their wind geometries and assess the similarities among the WN subclass. Characterizing the mass loss and transfer structures within these systems will help to constrain the future evolution of these WN stars and their roles as LGRB prognitors