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Aims. We perform a study of the structure, density, and distribution of ionized circumstellar gas in the strongly interacting binary SX Cas. Methods. We apply our new model codes for electron scattering in circumstellar matter to analyze a previously unpublished, extensive linear polarization data set for SX Cas, collected during four successive observing periods in 1981–84 at the Crimean Astrophysical Observatory. These data are complemented by our polarization observations carried out in 2000 and 2005 with the KVA-60 telescope at La Palma. Results. A clear, phase-locked pattern of polarization variations over the 36.6-day orbital period is revealed in each season, showing significant 1st and 2nd harmonic Fourier components and pronounced effects in the U and B bands at the primary eclipse, when the hot component and the surrounding circumstellar matter are obscured by the companion star. Seasonal variability is also found, but the major features of the polarization curves are well explained by a model with an extended scattering region on the trailing side of the accreting star, where the stream from the companion hits the accretion disk. No significant polarized flux is detected from the disk. We interpret this as being due to multiple scattering and strong absorption effects in the optically thick medium, which reduce the polarization of the light emerging from the disk to low levels. The modeled scattering region is above the orbital plane by about 10 ◦ ,a s seen from the center of the mass gainer. We attribute this to dominant electron scattering taking place on the upper side of an optically and geometrically thick accretion disk seen at an inclination i ∼ 78 ◦ by the observer.

UBVRI polarimetry of the massive interacting binary SX Cassiopeiae: modeling the electron-scattering circumstellar envelope