  We describe a method of determining the system parameters in non-eclipsing
interacting binaries. We find that the extent to which an observer sees the
shape of the Roche-lobe of the secondary star governs the amount of distortion
of the absorption line profiles. The width and degree of asymmetry of the
phase-resolved absorption line profiles show a characteristic shape, which
depends primarily on the binary inclination and gravity darkening exponent. We
show that, in principle, by obtaining high spectral and time resolution spectra
of quiescent cataclysmic variables or low mass X-ray binaries in which the
mass-losing star is visible, fitting the shape of absorption line profiles will
allow one to determine not only the mass function of the binary, but also the
binary inclination and hence the mass of the binary components.
