Ultraviolet observations of the 1980 eclipse of the symbiotic star CI Cygni

Secular and eclipse variations of UV lines and continua during the course of nearly a full orbit of the symbiotic binary Cl Cygni are presented. High-excitation resonance lines have brightened on an orbital time scale and show minimal effects of eclipse, while intercombination lines have faded and show pronounced but nontotal eclipse effects. The data are discussed in terms of mass transfer from the extended cool envelope of the red giant to a compact secondary. The formation of an accretion disk is a transitory phenomenon in which viscosity eventually dissipates the disk over orbital time scales. A large-scale low-density nebula explains the intercombination line emission, whereas the resonance line emission apparently arises in a large volume emitting region, possibly formed through shock collision from interacting stellar winds from the primary and secondary. Mass transfer rates are briefly discussed. Subject headings: stars: combination spectra— stars: eclipsing binaries — stars: mass loss — stars: winds