Multicolor photometry of CH Cygni in 1978-1998

We present the results of long-term (1978-1998) infrared and optical observ ations of the unique symbiotic system CH Cygni. The system's IR brightness and color variations are generally consistent with a model in which the sou rce is surrounded by a dust envelope with variable optical depth. There was evidence for a hot source in the CH Cyg system during the entire period fr om 1978 to 1998, with the exception of several hundred days in 1987-1989. O ver the observation period, there was tendency for the system to gradually redden at 0.36-5 mu m, accompanied by a brightness decrease at 0.36-2.2 mu m and a brightness increase at 3.5 and 5 mu m. The "activation" of the cool sources in 1986-1989 nearly coincided with the disappearance of radiation from the hot source. The dust envelope of CH Cyg is not spherically symmetr ical, and its optical depth along the line of sight is substantially lower than its emission coefficient, the mean values being tau(ex)(L) similar to 0.06 and tau(em)(L) similar to 0.16. We confirm the presence of a 1800- to 2000-day period in both the optical and IR, both accounting for, and not ac counting for, a linear trend. The spectral type of the cool star varied bet ween M5III and M7111. The spectral type was M5III during the phase of maxim um activity of the system's hot source, while the spectral type was M7III w hen the star's optical radiation was almost completely absent. The luminosi ty of the cool giant varied from (6300-9100) L-circle dot; its radius varie d by approximately 30%. The ratio of the luminosities of the dust envelope and the cool giant varied from 0.08 to 0.5; i.e., up to 50% of the cool sta r's radiation could be absorbed in the envelope. The temperature of dust pa rticles in the emitting envelope varied from 550 to 750 K; the radius of th e envelope varied by more than a factor of 2. The expansion of the emitting dust envelope observed in 1979-1988 accelerated: its initial velocity (in 1979) was similar to 8 km/s, while the maximum velocity (in 1987-1989) was similar to 180 km/s. Beginning in 1988, the radiation radius of the dust en velope began to decrease, first at similar to 45 km/s and then (in 1996-199 8) at similar to 3 km/s. From 1979 until 1996, the mass of the emitting dus t envelope increased by approximately a factor of 27 (the masses in 1979 an d 1988 were similar to 1.4 x 10(-7) M-circle dot acid similar to 3.8 x 10(- 6) M-circle dot, respectively), after which (by 1999) it decreased by nearl y a factor of 7. The mass-loss rate of the cool star increased in 1979-1989 , reaching similar to 3.5 x 10(-6) M-circle dot/yr in 1988. Subsequently (u p to the summer of 1999), the envelope itself began to lose mass at a rate exceeding that of the cool star. The largest input of matter to the envelop e occurred after the phase of optical activity in 1978-1985. If the envelop e's gas-to-dust ratio is similar to 100, the mass of matter ejected in 1988 was similar to 4 x 10(-4) M-circle dot. (C) 2000 MAIK "Nauka/Interperiodic a".