On the nature of CI Cygni.

Spectroscopic observations of CI Cyg, obtained at 3400-5100 A with resolution about 3 A using the Canadian Copernicus Spectrograph on the 90-cm Schmidt-Cassegrain telescope at Torun Observatory during 1981-1983, are reported. The results are presented in tables and graphs and characterized in detail. Consideration is given to nontotal-eclipse effects in H and He (but not in forbidden lines such as O III, Ne III, Fe VII, and N V); the physical parameters and geometry of the line-forming regions; the upper limit of He/H abundance (0.236 + or - 0.009); the accretion process (at rate 0.00005-0.0001 solar mass/yr); and the history of CI Cyg.