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 \IBVShead{4368}{2 September 1996}

 \title{CH Cygni -- a tenth magnitude star!}
 
\begintext

CH Cygni is a peculiar binary system in which a magnetic white dwarf
accretes matter from the M giant's wind moving on a long period highly
eccentric orbit (Miko{\l}ajewski et~al.  1996 and references therein).
It is known as the brightest symbiotic star.  During 1981--1984 its V 
brightness was about 5\fmm, and the star was seen even by the naked eye.  
Recently, its
brightness dropped significantly and reached almost 10\fmm\ in visual,
the dimmest value ever observed.  Several periods of high activity
separated by quiet ones have been observed since 1963.  During the
outbursts a hot F--A continuum and numerous emission lines dominated in
the UV and optical.  The late M giant in the system dominates the
spectrum between active phases and shows different kinds of variability.



Photoelectric UBVRI and UBV observations were continued using the 60cm
and 48cm telescopes at Toru\'n and Tartu observatories, respectively
(see Leedj\"arv, Miko{\l}ajewski 1995, Miko{\l}ajewski et al.  1992).
The UBVRI light curves covering the last decade are shown in
Figure~1.  After the 1985 eclipse (Miko{\l}ajewski et al.  1987), the
star reached a quiet inactive state by the end of 1987, during which a
pure M giant's 100\fdd\ pulsations with an amplitude of 0\fm7 started to
dominate the BV bands.  During 1989--1991 the star  showed some
transitory (1--3 months) episodes of activity clearly visible in U
light.  Starting from the early 1992   a relatively
strong and long activity period has been developing  with two gaps in the ends of 1992 and
1994.  An additional F--type supergiant continuum from the hot component
(Kuczawska et al.  1992) has modified the light curves in all
wavelengths.  Kuczawska et al.  have also demonstrated evidence of a
flickering down to  the RI bands in July 1992.



In the mid--1995 the hot continuum dropped rapidly. During the
last three-four 100\fdd\ pulsations of the M giant, which are of a similar
shape and amplitude as were previously observed between 1987 and 1990
(Miko{\l}ajewski et al.  1992), those pulsations  became the 
main features in
the BV light curves.


Another pronounced variability of the M giant is probably connected
with occupation of the star's surface by large cool spot (or spots) and
the rotation of the star with a period of about 760 days 
(Miko{\l}ajewski et al.
1992). We have found all observed broad minima of this periodicity using
all available photoelectric data (Miko{\l}ajewski et al. 1990, 1992
and references therein). In BV filters minima were observed during
inactive phases, only. Only two out of 12 minima (Table 1) are not compatible with this periodicity. At least the last one (JD\,2449785) is
probably caused by drop of earlier observed F--type continuum of the
active component. From the other minima in Table~1 we have obtained the
following ephemeris:

\hspace*{3cm}JD Min(760) = 2446549 + 746.8$\times$E\hfill (1)
\hspace*{71mm}$\pm$30\ \ \ \ \ $\pm$3.5
\vfill \eject

\centerline{2}
\vskip 1mm
The O$-$C values for this ephemeris are also listed in Table~1. The
747\fdd\ period is practically the same as the value of 749 days found by
Hinkle et al. (1993) which they interpreted as an orbital period of an
inner symbiotic pair on eccentric orbit in their triple--star model of
CH Cygni.



\begin{center}
Table 1. Observed minima of the 747\fdd\ periodicity
\begin{tabular}{ccccccccc}
\hline
E & Min & Filters & O\,$-$\,C & &E & Min & Filters & O\,$-$\,C \\
  &JD2400000+&  & days & & &JD2400000+&  & days  \\
\hline
$-$9  & 39820  & BV & $-$8 & &0   & 46535  & VRI&  $-$14 \\
$-$8  & 40535  & BV & $-$40 & &1   & 47295  & BVRI& $-$1 \\
(?)& (41135)& BV & ($-$186) & &2   & 48030  & BVRI& $-$13 \\
$-$7  & 41355: & BV & +34 & &3   & 48735  & RI  & $-$54 \\
$-$6  & 42080  & BV & +12 & &4   & 49595  & RI  & +59 \\
$-$5  &42840   & BVI& +25 & &(?) & (49785)& RI  &(+249) \\
\hline
\end{tabular}
\end{center}

\epsfysize=14cm
\centerline{\epsffile{4368-f1.ps}}
\begin{center}
 Figure 1. The UBVRI light curves of CH Cygni
including the last minimum in June 1996.  In the lowest box the
calculated minima from ephemeris (1) are marked.
\end{center}
\vfill \eject

\centerline{3}
\vskip 1mm

Apart from an increasing period of pulsation, Miko{\l}ajewski et al.
(1990, 1992) have found  another evidence of a luminosity pulse
produced by a helium--flash on the AGB for the M giant in CH Cyg --- 
a systematic decrease in the optical light on the time scale of
hundred years.  The rate was estimated to be  about 0\fm60/100yr in the
visual from historical data, and seems to achieve at least 1\fmm/100yr
during the last decade.

In conclusion, the minimum of brightness observed in June 1996 is a
result of (1) inactivity of the hot component, (2) secular decline of
the M giant's brightness, and (3) coincidence of the minima of
747\fdd\ periodicity (rotation) and 100\fdd\ pulsation.  In Figure~2 we
show V magnitudes, together with U$-$B and B$-$V indices during last two
years.  We have marked two last minima with epoch $E=4$ and $E=5$
calculated from Eq.~1 and last four (from $e=87$ to $e=90$) minima
predicted by the quadratic ephemeris:
\vskip 1mm
\hspace*{3cm}JD Min(100) = 2440935.3 + 102.41e + 0.0140e$^{2}$\hfill(2)
\vskip 1mm

\noindent
for the M giant's pulsations (Miko{\l}ajewski et al.  1992).  The last
three pulsation minima are evidently visible in the V light.  Observed
minima  confirm the systematic increase of the
pulsation period, which reaches now almost 105\fdd.  Taking into account
the secular decrease of brightness in the optical we can predict from
Eqs.  (1) and (2) the next coincidence of minima in July 1998, when an
even deeper minimum should be observed (if the hot component remains
inactive).


\begin{figure}[hpt]
        \begin{minipage}{7.9cm}
                \epsfxsize7.5cm \begin{center} \leavevmode \hspace*{-5mm}
                \epsfbox{4368-f2.ps} 
                \end{center}
        \end{minipage}
        \begin{minipage}{7.9cm}
                \epsfxsize7.5cm \begin{center} \leavevmode
                \epsfbox{4368-f3.ps} 
                \end{center}
        \end{minipage}
\vspace{2ex}
\end{figure}






\begin{minipage}[t]{0.47\textwidth}
Figure 2. The V light curve and U$-$B and B$-$V indices of CH Cyg
during last 2.5 years. Last epochs of minima from ephemerides (1) and
(2) are marked as {\it E} and {\it e}, respectively.
\end{minipage}\hfill
\begin{minipage}[t]{0.47\textwidth}
Figure 3. The [OIII]\,4959\,\AA\ and 5007\,\AA\, region in the spectrum
of CH Cygni.
\end{minipage}



\vfill \eject

\centerline{4}
\vskip 1mm
Spectral observations during the last three years were carried out
with a CCD--camera mounted in the coud\'e-spectrograph of the 2m
telescope at NAO Rozhen.  In Figure~3 the behaviour of the nebular lines
of [OIII]\,4959\,\AA\ and 5007\,\AA\, as well as the changes in the hot
veiling continuum, during the drop of star's brightness in May--June 1996
are shown.  It is obvious, that the flux in the continuum decreases
during all the time and that the [OIII] nebular lines reached a maximum
intensity coinciding with the light minimum in June 1996. At the same time,
a nebula extended to about 30\farcss\ was observed in the optical by
Corradi \& Schwarz (1996).  Variations in the intensity, profile shapes
and strong dependence on the slit orientation for the nebular lines of
[OIII] were observed in 1994 (Tomov et al.  1996).  So, it is evident
that there is a significant amount of relatively low density matter
around the CH Cyg system, which may be responsible for the origin of the
nebular lines.




{\it Acknowledgement:} This research was sponsored by the Polish KBN
Grant No. 2 P304 007 06, Bulgarian NFSR Grant No. F--574/1995 and the
Estonian Science Foundation Grant No. 827/1996.

\vskip 1cm

\beginauthors
\authorsrightcol
M.~MIKO{\L}AJEWSKI
Institute of Astronomy
Nicolaus Copernicus University
Chopina 12/18, 87-100 Toru\'n,
Poland
e-mail: mamiko@astro.uni.torun.pl
\vskip 2mm
T.~V.~TOMOV, D.~KOLEV
NAO Rozhen
P.O.Box 136
4700 Smolyan, Bulgaria
e-mail: rozhen@tempus.tu-plovdiv.bg
\vskip 2mm
L.~LEEDJ\"ARV
Tartu Astrophysical Observatory
EE2444 T\"oravere, Estonia
e-mail: leed@aai.ee
\endauthors

\references

Corradi, R., Schwarz, H., 1996,  {\it Physical
Processes in Symbiotic Binaries and Related Systems}, ed. J. Miko{\l}ajewska, 
Foundation for Polish Astronomy, Warsaw, in press

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

Kuczawska, E, Miko{\l}ajewski, M., Kirejczyk, K., 1992, {\it IBVS},
No.\,3806

Leedj{\"a}rv, L., Miko{\l}ajewski, M.: 1995, {\it A\&A}, {\bf 300}, 189

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

Miko{\l}ajewski, M., Miko{\l}ajewska, J., Khudyakova, T.N., 1990, {\it
A\&A}, {\bf 235}, 219

Miko{\l}ajewski, M., Miko{\l}ajewska, J., Khudyakova, T.N., 1992, {\it
A\&A}, {\bf 254}, 127

Miko{\l}ajewski, M., Miko{\l}ajewska, J., Tomov, T., 1996, 
{\it Compact Stars In Binaries}, eds. J. van
Paradijs et al., Kluwer, Dordrecht, p.451

Tomov, T., Kolev, D., Munari, U., Antov, A., 1996, {\it MNRAS}, {\bf
278}, 542


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