UBV-JHKLM photometry of CH Cygni over 1978-1995: dust properties and doubts on the triple star model.

We present new UBV-JHKLM photoelectric photometry of the symbiotic binary 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 conditions 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 sinusoid of 32year period. It may be a dust obscuration event similar to those 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 properties of the cool giant denounce a clear partnership with the spheroidal component of the Galaxy. This lowers the estimated distance to ~120pc and the cool giant mass to ~1.0M_sun_. 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 ~1,000K. The condensed dust grains 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 outbursting white dwarf. The recently proposed triple-star model for CH Cyg is confronted with photometric observations. Several serious discrepancies are outlined and individually discussed. We believe that, without additional evidences and careful modelling, the triple star model cannot survive the comparison with the photometric observations. The low amplitude (2.6km/s) and periodic (756days) radial velocity variations apparently do not trace an orbital motion. They may be due to one of the many superimposed pulsation modes of the highly variable M giant.