COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3742 Konkoly Observatory Budapest 6 July 1992 HU ISSN 0374 - 0676 CH Cygni in 1992: the strongest activity after the large outburst in 1977-1986 CH Cyg is a symbiotic binary consisting of an M giant semiregular variable and a white dwarf probably possessing strong magnetic field (Mikolajewski et al. 1990). Since 1989, the star has shown some episodes of erratic activity (e.g. Mikolajewski et al. 1990; Leedjarv 1990; Bode et al. 1991; Tomov and Mikolajewski 1992). In particular, two transitory (approx. 1-2 months) maxima of UBV color indices related to reappearance of the hot continuum were. observed in 1989, and two more pronounced events happened in 1990-1991 (Fig. 1). The periods of increased brightness were also accompanied by flickering activity and remarkable changes in the emission line spectrum. [FIGURE 1] Figure 1. The UR light curves of CH Cyg during 1985-1992, mostly based on data obtained at Tartu, Torun and Byurakan Observatories (Tomov et al. 1992, in preparation). In the beginning of 1982, the hot continuum started to grow up again. In March-April 1992, the U magnitude was about 8.5m, which means that the hot continuum was as strong as that observed in 1986, shortly before the end of the 1977-1986 outburst (Fig. 1). At the same time, the cool giant was close to minimum of its 770d variability as Indicated by low RI magnitudes (R approx. 5.6m; I approx. 3.2m), and spectral type between M7 and M8 derived from the VO absorption (Bode et al. 1991). Mikolajewski, Mikolajewska and Khudyakova (1992) have recently suggested that this variability can be due to rotation of the giant's photosphere covered with a large dark spot. Although the hot component brightness is comparable to that observed in 1986, the very low brightness of the M giant causes the spectrum in the UB range to be dominated by the hot component. In fact, even the most prominent spectral features of the M giant (e.g. Cal 4227 A and TiO bands) are practically fully veiled by the hot continuum (Fig. 2), and the flickering variability characteristic of the hot source is visible in U band (Fig. 3) and even in BV bands. The optical spectra taken in 1992 were characterized by very bright HI Balmer lines, strong He I and relatively weak [Fe II] and Fe II emission lines (e.g. comparing with 1986, when the overall intensity of the hot spectrum was comparable), as well as the presence of forbidden [O III], [Ne III] and [S II] lines. Ca II "H" and "K" emission lines with core absorptions were stronger than anytime before. It was surprising that H_8 had similar profile, with blue-shifted absorption component of increasing strength. Unfortunately, we do not have any idea about the origin of this strange H_8 profile. All remaining HI Balmer lines had single component profiles without any trace of absorptions. Between January and April 1992, intensities of the Balmer emission lines and continuum as well as of the other permitted lines have been growing up, while the forbidden line fluxes have been slightly decreasing or constant (Fig. 2). [FIGURE 2] Figure 2. Coude-spectra (0.35A) of CH Cyg obtained at Rozhen Observatory. [FIGURE 3] Figure 3. Flickering in U light, observed with two-channel photometer and 60cm telescope at Mt. Suhora Observatory. Rapid fluctuations with an amplitude of 30 - 50% and a timescale of minutes are clearly visible. The erratic activity of the hot component in 1989-1992 was unrelated to the M giant behavior. In particular, the 1989 summer episode coincided with maximum brightness of the giant, while the present one occured during its minimum, This activity rather results from highly unstable physical conditions near the white dwarf. After the jet ejection in 1984, the accretion complex around the magnetic white dwarf was partially disrupted (Mikolajewski, Mikolajewska and Kudyakova 1990). At present, matter accreted from the giant's wind falls directly onto magnetosphere, causing the observed phenomena. In fact, strong HI Balmer emission presence of nebular [O III] and [Ne III] emission, and especially the profiles of prominent Ca II "H" and "K" emissions, indicate strong outflow of matter, which can be due to propeller interaction of the rapidly rotating magnetic white dwarf with the giant's wind (see Mikolajewski et al. 1990 for details of the model). This project was partly sponsored by the Bulgarian NFSR under contract F-35 and the Polish KBN grant No. 2/1182/91/01. M. Mikolajewski^1, T. Tomov^2, E. Kuczawska^3, L. Leedjarv^4, J. Mikolajewska^1, B. Wikierski^1,5 1 Institute of Astronomy, Nicolaus Copernicus University, ul. Chopina 12/18, Pl-87100 Torun, Poland 2 National Astronomical Observatory Rozhen, P.O. Box 136, 4700 Smoljan, Bulgaria 3 Institute of Physics, Pedagogical University, ul. Podchorazych 2, Pl-30084 Krakow, Poland 4 Institute of Astrophysics and Atmospheric Physics, Tartu-Toeravere, 202444 Estonia 5 Nicolaus Copernicus Astronomical Center, ul. Chopina 12/18, Pl-87100, Torun, Poland References: Bode, M.F., Roberts. J.A., Ivison, R.J., Meaburn, J., Skopal. A.: 1991, MNRAS 253, 80 [BIBCODE 1991MNRAS.253...80B ] Leedjarv, L.: 1990, IBVS No. 3474 Mikolajewski, M., Mikolajewska. J., Khudyakova. T.N.: 1990, Astron. Astrophys. 235, 219 [BIBCODE 1990A&A...235..219M ] Mikolajewski, M., Mikolajewska. J., Khudyakova. T.N.: 1992, Astron. Astrophys. 254, 127 [BIBCODE 1992A&A...254..127M ] Mikolajewski, M., Mikolajewska. J., Tomov, T., Kulesza, B., Szczerba, R., Wikierski, B.: 1990, Acta Astr. 40, 129 [BIBCODE 1990AcA....40..129M ] Tomov, T., Mikolajewski, M.: 1992. IBVS No. 3721