Broadband Polarimetric Light Curves of AO Cas

Because the majority of massive stars interact with binary companions, characterizing the resulting mass and angular momentum loss is key to understanding their subsequent evolution. AO Cas is a contact binary, a unique, short-lived phase where both of its O stars are filling or nearly filling their Roche lobes. It also shows evidence of colliding winds. Recently, we have begun analysis of 17 previously unpublished, archival spectropolarimetric observations of AO Cas. Our preliminary results show that AO Cas exhibits strong, phase-locked variations in its broadband polarization behavior, but that its lines do not appear to be polarized differently than its continuum. We will discuss these data with emphasis on their implications for the system’s contact binary and colliding wind geometry.