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\IBVShead{4157}{8 February 1995}

\centerline{\ft PHOTOMETRIC OBSERVATIONS OF ECLIPSES IN}
\centerline{\ft THE SYMBIOTIC TRIPLE SYSTEM CH~CYGNI}
\vskip 8mm

\begintext

CH~Cygni is probably the most peculiar object among all known symbiotic
stars. The complex behaviour of the spectrophotometric parameters and the
light curve observed during the outbursts (1963, 1967--1970, 1977--1986)
led, at first, to a single star model (e.g. Faraggiana \& Hack 1971).
The binary nature of CH~Cyg was suggested by radial velocity measurements
in the {\em optical} spectrum for both components (e.g. Yamashita \& Maehara
1979, Tomov \& Luud 1984) and by the eclipse of the hot component by the
red giant observed during 1985 May -- October (e.g. Miko{\l}ajewski et al.
1987). CH~Cyg then was accepted as the symbiotic binary with the long 5700-day
period orbit. Recently Hinkle et al. (1993) analysing a 13-year time series
of high-resolution 2$\mu$m {\em infrared} spectra of CH~Cyg, found a
very regular variation in the red giant radial velocities with a 756-day
period. They suggested that CH~Cyg is a triple system with the symbiotic
pair being the short 756-day period system, while the unseen G-K dwarf
revolves around the symbiotic binary in the long 14.5-year period orbit.
However they state that CH~Cyg cannot be an eclipsing system, and the
orbital inclination must be $\sim 70^{\circ}$. The aim of this
contribution is to present our new photometry showing that CH~Cyg really
is an {\em eclipsing} symbiotic triple system. 

CH~Cyg has been regularly monitored at the Skalnat\'{e} Pleso (1750\,m above the
sea level) and Star\'{a} Lesn\'{a} (890, our down station near Tatransk\'{a}
Lomnica) observatories. The observations have been made in the standard UBV
system using a one-channel photoelectric photometer installed in the Cassegrain
focus of the 0.6/7.5\,m reflectors. The stars HD\,182\,691 (V=6.525,
B$-$V=$-$0.078, U$-$B=$-$0.24) and SAO~048\,428 (m$_{\rm v}$=8.0, m$_{\rm pg}$=8.6,
spectrum F8) were used as the comparison and the check stars, respectively. 

In Figure 1 we show the historical U-light curve as this represents best
behaviour in the blue continuum of CH~Cyg during outbursts, quiescence as well
as eclipses. Our new data observed up to 1995 January 31 are displayed
in more detail in Figure 2 together with those published by Panov \& Ivanova
(1992) and Leedj\"{a}rv (1993).
It covers the period of the current outburst which began at
the beginning of 1992 (Skopal et al., 1992). During this active
phase photometric observations indicate the two deep minima of about
2 and 3\,mag in the U band, respectively, and times of their centres
at JD~2\,448\,922 and JD~2\,449\,685$\pm$5. Other such minima occurred
during the 1967--1970 active phase at $\approx$JD~2\,439\,810 and
JD~2\,440\,543. Positions of these four minima agree well with times of the
spectroscopic conjunction of the symbiotic pair (cool component in front)
in the triple-star model of CH~Cyg -- $T_{\rm conj}$ = JD~2\,445\,888,
$P_{\rm orb}$ = 756$\pm$4 day, circular orbit -- derived by Hinkle et al.
(1993). Thus we can conclude that these minima are caused by the eclipse
of the active component by the cool component in the {\em short-period}
symbiotic pair. 
\vfill \eject

\centerline{2}
\epsfysize=9cm
\centerline{\epsffile{4157-f1.ps}}
\centerline{Figure 1. Historical U-light curve for CH~Cyg. Eclipses}
\centerline{in both the long- and short-period orbit are marked.}

\epsfysize=9cm
\centerline{\epsffile{4157-f2.ps}}
\centerline{Figure 2. The UBV light curves of CH~Cyg covering the current outburst.}
\centerline{The minima correspond to the eclipses of the active component} 
\centerline{by the cool component in the 756-day period symbiotic pair.}

\vskip 3mm
During the 1970--1977 and 1986--1991 quiescent periods, due to a very
low level of the blue continuum, the eclipses could not be detected.
Nor were they present in the light curve during the last,
1977--1986, active phase. Due to a very high level of activity during
that period, the circumstellar matter in the symbiotic pair was probably
located within the common potentials of the binary, and that way, instead
of deep and relatively narrow minima, we observed a rather complex
wave-like variation in the light curve (cf. Figure~1).
\vfill \eject

\centerline{3}
\vskip 1mm

On the other hand the periodic $\sim$756-day variation in the
U, B, V light curves (cf. Skopal 1989) at a level of about 1.5\,mag and
with minima at the spectroscopic conjunction observed during the quiescent
periods, resembles that present in light curves of the other classical
symbiotic stars (e.g. AG~Peg, V443~Her, AG~Dra, EG~And).

In addition, the historical light curve exhibits two more minima in
the U-band at $\sim$JD~2\,441\,130 and JD~2\,446\,275$\pm 75$. Their positions
agree perfectly with times of the spectroscopic conjunction in the
{\em long-period} orbit -- $T_{\rm conj}$ = JD~2\,446\,346$\pm 340$,
\hfill $P_{\rm orb}$=5298

\noindent $\pm 98$ days, $e=0.067$, $\omega=207^{\circ}$
(Hinkle et al. 1993). Bearing in mind the very large separation of
the components in the long-period orbit, approximately of 1700\,$R_{\odot}$,
and the radius of the giant to be of 180 to 200\,$R_{\odot}$, the orbit
inclination must be very high to produce the observed shape of the
minima -- mainly that in 1985. A rough estimation gives $i > 83^{\circ}$.

The agreement between the positions of the minima observed in the U-light
curve and times of the spectroscopic conjunctions for both orbits allow us
to conclude that CH~Cyg is an {\em eclipsing} symbiotic triple system.


\vskip 1cm

\beginauthors
\authorsrightcol
A. SKOPAL
Astronomical Institute,
Slovak Academy of Sciences,
059~60 Tatransk\'{a} Lomnica
Slovakia
\endauthors

\references

Faraggiana R., Hack M., 1971, {\it A\&A}, {\bf 15}, 55

Hinkle K.H., Fekel F.C., Johnson D.S., Scharlach W.W.G., 1993, {\it AJ}, {\bf 105}, 1074

Leedj\"{a}rv L., 1993, {\it IBVS}, No.\,3951

Miko{\l}ajewski M., Tomov T., Miko{\l}ajewska J., 1987, {\it Ap\&SS}, {\bf 131}, 733

Panov K.P., Ivanova M.S., 1992, {\it IBVS}, No.\,3817

Skopal A., Hric L., Kom\v{z}\'{\i}k R., 1992, {\it IAU Circ.}, 5504

Skopal A., 1989, {\it Contrib. Astron. Obs. Skalnat\'{e} Pleso}, {\bf 18}, 31
 
Tomov T., Luud L., 1984, {\it Afz}, {\bf 20}, 99

Yamashita Y., Maehara H., 1979, {\it PASJ}, {\bf 31}, 307

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