The symbiotic star CI CYG : S-process episode or accretion event ?

Evidence that the symbiotic star C I Cygni is an eclipsing binary is reviewed. It is shown that the s-process episode, described by Audouze et al. (1981) during its 1975 outburst, arises from superposition of normal gM4 absorption features on the continuum of the hot component during eclipse ingress, and not from sudden enhancements of rare earth elements. The peculiar velocity displacements of absorption lines with different excitation potentials during this episode are identified as signatures of an optically thick accretion disk, which dominates the visible spectrum during outburst. The data presented by Audouze et al., and the shape of the light curve thus provide evidence that the outburst is accretion-powered.