IUE low - resolution observations of the triple system CH Cygni : the mass transfer in the symbiotic pair ?

We present and analyse UV low-resolution spectra of CHCyg taken during the whole period of the International Ultraviolet Explorer (IUE) mission. Our analysis of the variation in the UV/optical hot star continuum confirmed the basic scenario of sporadic accretion in the symbiotic pair of the triplestar system as being responsible for maintaining its activity. During active phases, the hot continuum consists of two components. The first, far-UV emission, can be associated with a region where the material from the giant star in the symbiotic pair impacts the accreting matter around the hot star. The second, seen in the near-UV (λ ≥2200 Å) and the optical UBV region, can be produced by the accretion process onto the central star. Emission here shows characteristic temperatures of 6500 – 8700K. Both sources require an equivalent mass transfer rate of 1 − 2 × 10−5M yr−1 during the 1979-84 maximum, and about of 2×10−6M yr−1 in the 1992-95 active phase for derived parameters of the hot component in the triple-star system. Variation in the luminosities of both sources suggests that the basic physical principles of mass transfer in CHCyg are similar to those of cataclysmic variables (CVs).