This thesis presents a detailed theoretical and observational study of the polarization arising from single massive stars and massive binaries. To derive stellar parameters such as mass-loss rates, the amount of atmospheric asymmetry and inclination angles, we have developed and utilized sophisticated numerical radiative transfer models, which compute both the stellar spectrum and the polarization, and applied them to the analysis of observational data.;As a case study of the geometry of a single Wolf-Rayet star, the polarization variation across the strong C IV (lambda5805) emission line of WR 111 (a WC5 star) has been modeled. We fit the observational data with the models by using the continuum polarization as a constraint and by treating the interstellar polarization as a free parameter instead of using unreliable values of interstellar polarization estimated from analysis of field stars. Our results suggest that the global deviation from spherical symmetry of this object is no larger than 20%, and it is probably less than 10%. This indicates that the rotation of the WC star is relatively slow and unimportant for its mass-loss process.;A 3-D Monte Carlo radiative transfer model, which computes line and continuum polarization variability for a binary system with an optically thick non-axisymmetric envelope, was developed. An 8-way tree data structure was constructed to achieve high precision with efficient use of computer resources. The model can be adapted easily to a system with an arbitrary density distribution and large density gradients.;Our model was used, in conjunction with the multi-line non-LTE radiative transfer model of Hillier, to determine the mass-loss rate of the Wolf-Rayet star in the massive binary V444 Cyg (WN5 + 06). The observed He I (lambda5876) and He II (lambda5412) line profiles, and the V band continuum light curves of three Stokes parameters (I, Q, U) were simultaneously fitted with our models. The mass-loss rate determined, M WR = 0.6 (+/-0.2) x 10-5 M ⊙ yr-1, is more accurate than earlier values because it is based on many independent observational constraints. It provides an important constraint for stellar evolution calculations.
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