UBV-JHKLM PHOTOMETRY OF CH-CYGNI OVER 1978-1995 - DUST PROPERTIES ANDDOUBTS ON THE TRIPLE STAR MODEL

We present new UBV-JHKLM photoelectric photometry of the symbiotic bin ary CH Cyg covering the period 1991-1995, which extends our monitoring started in 1978. The large and highly homogeneous set of data that we have accumulated in the last eighteen years is reviewed and discussed . By July 1995 the outbursting component has returned to the same cond itions which characterized the previous minimum in 1988-1989. In J,H,K CH Cyg shows a long term modulation that can be fitted with a sinusoi d of 32 year period. It may be a dust obscuration event similar to tho se known to undergo in symbiotic Miras. The cool giant exhibits in the infrared a variability of large amplitude, best described as chaotic- like. The only detectable periodicity is 1980 days. The photometric pr operties of the cool giant denounce a clear partnership with the spher oidal component of the Galaxy. This lowers the estimated distance to s imilar to 120 pc and the cool giant mass to similar to 1.0 M. Several episodes of dust condensation in the wind of the giant are identified. One is in full progress at the time of writing. The dust condensation temperature is found to be similar to 1,000 K. The condensed dust gra ins absorb selectively in the infrared but are large enough to absorb neutrally in the UBV wavelength region. There is no evidence for dust condensing in an hypothetical wind or ejected material from the outbur sting white dwarf. The recently proposed triple-star model for CH Cyg is confronted with photometric observations. Several serious discrepan cies are outlined and individually discussed. We believe that, without additional evidences and careful modelling, the triple star model can not survive the comparison with the photometric observations. The low amplitude (2.6 km sec-1) and periodic (756 days) radial velocity varia tions apparently do not trace an orbital motion. They may be due to on e of the many superimposed pulsation modes of the highly variable M gi ant.