The UV variability of BF Cygni.

BF Cygni is one of the symbiotic systems in which orbital effects are better seen. The UV continuum and emission lines undergo large variations, in good agreement with the predictions of optical photometry. The strong changes in the shape of the ultraviolet continuum suggest the existence of a strongly wavelength dependent opacity. At least at some phases this is due to Rayleigh scattering by the extended atmosphere of the cool giant. Emission lines are most likely affected also by these opacity effects and therefore their variations cannot be used to study the changes in the physical conditions along the orbit. The long duration of the eclipses indicates that they are not only due to occultation by the red giant companion, but also by its extended envelope. The depth of the eclipse suggests a large density in the giant envelope, and therefore, a high mass loss rate. The ultraviolet data indicate that the ionized region should be cone-shaped around the hot star in order to explain the long duration of the eclipses.