Spectroscopy of the symbiotic binary CH Cygni from 1996 to 2007

Context. Efforts to uncover the nature of the variable star CH Cyg have lasted for decades by now. Recently, the model of a red giant and an accreting white dwarf on a similar to 5700-day orbit has seemed to have gained an advantage over other ideas, mainly as a result of the investigation of the cool component's absorption lines. In addition to those lines the star's spectrum also presents emission features, which form near the hot component and are therefore a probe of the accretion phenomenon, which in turn could be bound to the orbital motion. Aims. We have monitored a set of emission lines in the optical spectra of CH Cyg over an extended period of time to determine relations between the behaviour of those lines and the photometric data. We are searching for a possible connection with the star's orbital motion, and find hints of the physical phenomena occurring near the white dwarf. Methods. More than 500 observations of CH Cyg were carried out during 1996-2007 using the 1.5-m telescope at the Tartu Observatory, Estonia. Most of the spectra were registered in the H alpha region, but other Balmer lines and lines of He, N, O, and Fe were also investigated in terms of their equivalent widths, radial velocities, and absolute fluxes. Results. The spectra indicate different stages that CH Cyg has been through in the course of our observations. During quiescence, the strength of the line profiles changes in opposite phase with the star's luminosity. The H alpha profile becomes double-peaked at times, which may point to the temporary presence of some disk-like structure, but also to absorption in neutral gas surrounding the area of the formation of the emission component. Occasionally the line develops wide wings, which indicate high velocities of the matter. In 1999, when a periastron passage is assumed according to the 5700-day model, the Balmer lines strengthen considerably, as might be expected due to an increase in the accretion rate. The eclipse in 2001, however, is not noticed. At certain episodes, the controversial 756-day period of CH Cyg is seen in our data.