Polarization of UV radiation from accretion discs and the possible 39sup(d) period in cygnus X-1

UV radiation from accretion discs around compact X-ray sources may be linearly polarized due to scattering in the outer rim of the disc. The recently reported 39sup(d) modulation in the polarization of that radiation from Cyg X-l is interpreted as due to a modulation in the angular width of the rim, which results from variability in L1 overflow from the primary. The latter may be caused by tidal interaction with a third star moving in an eccentric orbit. The model can explain the available data in the U, B and V optical bands and makes further predictions, in particular with regard to the presence of similar modulations in other observables. (author)