UV eclipse observations of CI Cyg.

Low spectral resolution observations were obtained with the IUE during the eclipse phase. Additional data obtained by other IUE groups have been included in the eclipse observations, making it possible to examine the UV spectral properties of CI Cyg over nearly an entire orbit which spans early 1979 through mid 1981. Data obtained over this period suggest an overall decline in UV emission, consistent with the decline of optical emission following the outburst of 1975. The short-wavelength spectrum 1200-2000 A is characterized by numerous intense high-excitation emission lines which become more prominent out of eclipse. The LWR wavelength range 2000-3200 A exhibits a few more additional lines of O III, Mg II, and He II which are superimposed on continuum that rises gradually with increasing wavelength. The observations are consistent with a binary star model which involves mass transfer from the extended cool envelope of the primary to the compact secondary.