Variable polarization in X-ray binaries - HD 77581 (= 4U 0900-40)

Variable linear polarization has been observed from the X-ray binary system HD 77581 (= 4U 0900-40). These observations, together with observations of HD 153919 (= 4U 1700-37) and HD 152667, have been analyzed by the method of Brown et al. (1978). A polarimetric analysis should include only observations made over a time during which the scattering region producing the polarization does not change its structure. It is found that this time is typically on the order of 10 days for systems with OB supergiant primaries. An observation set compiled over a longer duration can introduce systematic errors into the derived parameters. The orbital and physical properties of HD 153919 and HD 152667 analyzed under these constraints are consistent with those derived from polarimetric observations taken 4 years earlier.