The long-period symbiotic binary CH Cygni. II. The M giant component : increasing pulsation period and spot-like activity.

We discuss multicolor photometry of CR Cygni obtained in 1986-1991, in terms of variability of the M giant. The giant pulsates radially in the first overtone with a period of ∼100 d and an amplitude of ∼O^m.5. The O-C diagram has been obtained for mean dates of minimum from intervals covering a dozen or so pulsation cycles using a linear ephemeris derived for the 100^d pulsation. The O-C's indicate a systematic increase of the pulsation period at a rate of about 5^d/100 yr. This phenomenon must be related to changes in the internal structure of the M giant. We are unable, however, to ascertain whether the systematic decrease in optical light reported in Paper I has the same cause as the period increase or it is due to increasing spot activity. Some arguments are given that the very deep minimum that occurred in the optical and infrared light in 1990 can be due to large cool spot(s) on the giant's photosphere.