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\IBVShead{4228}{14 August 1995}

\centerline{\ft PECULIAR OUTBURST BEHAVIOR OF GO Com}
% \centerline{submitted to IBVS}
\vskip 8mm

\begintext

  GO Com was discovered by Kowal (1977) as an eruptive object on a Palomar
plate taken on 1977 July 1.213.  The variable was confirmed to coincide
with a suspected variable star CSV\,1959 = SVS\,382 (Belyavskij, 1933).
On the other hand, Usher (1981) independently discovered a very blue star
of B=18.1 during the survey of the north galactic pole region.  This star
(US 31) was identified with GO Com.  The extreme color (U$-$B = $-$1.5) suggests
an extreme nature of this object.  Vogt and Bateson (1982) classified this
variable as a WZ Sge-type dwarf nova because of its large outburst amplitude
and low outburst frequency.


  The object has been visually monitored by VSOLJ members since 1986, and
the short outburst observed on May 30, 1989 (m$_v$=13.2) has been the only
record in recent years (Vanmunster and Howell 1995)
until Vanmunster (1995) detected another outburst
on July 16, 1995.  Although only single positive visual observation has
been available on this outburst, the outburst seems to have been confirmed
by our CCD observation on July 25 which caught the object at V=16.6.
A negative visual observation (fainter than 13.6) by M. Moriyama (private
communication) 1.5 day after Vanmunster's detection seems to indicate that
this outburst is also short-living just as one observed in May, 1989.


  14 days after this outburst, GO Com was unpredictably caught in outburst
on July 30.8, 1995 at m$_v$=13.3 (L. Szentasko, VSNET message).
This outburst was subsequently confirmed by our CCD observation.
We then started systematic time-resolved photometry to cover this outburst.
The observations were carried out using a CCD camera (Thomson TH~7882, 576
$\times$ 384 pixels) attached to the Cassegrain focus of the 60 cm
reflector (focal length=4.8 m) at Ouda Station, Kyoto University
(Ohtani et al., 1992).

  To reduce the readout noise and dead time, an on-chip summation of
2$\times$2 pixels to one pixel was adopted.
An interference filter was used which had been designed to reproduce the
Johnson {\it V} band.  The exposure time was between 60 and 180 s.
The frames were first corrected for standard de-biasing and flat fielding,
and were then processed by a microcomputer-based PSF and aperture
photometry package developed by one of the authors (T.K.).
The differential magnitudes of the variable were determined against
a local standard star (12\fh 56\fmm
36\fss 64 +26\degr 31\farcmm 42\farcs 9 (J2000.0), {\it V}=12.8). The
position and magnitude were taken from the Guide Star Catalog).
The constancy of this comparison was checked against several stars in
the same field.


  The resultant general light curve is given in Figure 1, which clearly
shows the long-living (lasting at least 7 days) nature of the present
outburst.  Existence of these two types (short and long) of outbursts
suggests the SU UMa-type nature of GO Com, as was first suspected by the
possible detection of 95 min periodicity in quiescent light curve
(Howell et al., 1990).  The rate of decline ($\sim$ 0.1 mag/day) also seems
to support that this outburst may be a superoutburst, probably first
documented one in GO Com.
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\centerline{2}
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\begin{center}
Figure 1.  A general {\it V}-band light curve of GO Com constructed
from all the CCD observations.  The zero point corresponds to {\it V}=12.8.
The outburst lasted at least 7 days showing a slow decline
($\sim$ 0.1 mag/day) followed by a rapid decline.
\end{center}
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\centerline{\epsffile{4228-f2.ps}}
\begin{center}
Figure 2.  Time-resolved photometry of GO Com on Aug. 5.  The light
curve seems to indicate a hump feature with an amplitude of 0.2 mag at
around Aug. 5.446 UT, which would be attributed to a superhump.
\end{center}
\vfill\eject

\centerline{3}
\vskip 1mm
  Due to unfavorable location in the sky, we can say little about existence
of superhumps which are characteristics to SU UMa-type dwarf novae.  The
data on Aug. 5 (Figure 2) seems to indicate a hump feature with an amplitude
of 0.2 mag at around Aug. 5.446 UT, which would be attributed to a superhump.
The secure classification of this dwarf nova should await for further
observations.


   Although the classification of GO Com is still immature, we should point
out the peculiar pattern of two consecutive outbursts.  As stated earlier,
the outbursts of GO Com have been very rare, likely one in few years.  Hence
the short interval (14 days) of these two outbursts is already a surprise.
Although combinations of normal and super-outbursts are sometimes observed
in SU UMa stars, they usually belong to either of the two patterns: (1) short
outburst following a superoutburst, (2) short outburst just preceding a
superoutburst.  In the latter case, the interval of the short and
superoutbursts are usually less than a few days.  Such ``precursor" type
short outburst is currently understood as a trigger of a superoutburst
in the scheme of thermal and tidal instabilities of the accretion disk
to explain dwarf nova outbursts (Osaki 1989).  Whether the current unique
pattern of outbursts of GO Com can be explained in the same scheme would
be an interesting problem. [Our latest observation indicates that GO Com
is again in outburst on Aug. 13.44 at V=15 -- 15.5.  This post-outburst
brightening can be fit by the pattern (1).  It is again interesting that
these two types of combinations of outbursts are consequently observed.]
\vskip 4mm
The authors are grateful to T. Vanmunster, L. Szentasko and G. Poyner
for their promptly circulating the news of this rare event through VSNET.
Part of this work is supported by a Research Fellowship of the Japan
Society for the Promotion of Science for Young Scientists (T.K).

\vskip 1cm

\beginauthors
\authorsrightcol
Taichi KATO
Daisaku NOGAMI
Hajime BABA
Kyoto University
Sakyo-ku, Kyoto 606-01 Japan
\endauthors


\references

Belyavskij, A. I., 1933, {\it Perem. Zvezdy}, {\bf 4}, 234

Howell, S., Szkody, P., Kreidl, T., Mason, K. O., Puchnarewicz, E. M., 1990, 
     {\it PASP}, {\bf 102}, 758

Kowal, C. T., 1977, {\it IAU Circ.},  2562

Ohtani, H., Uesugi, A., Tomita, Y., Yoshida, M., Kosugi, G., Noumaru, J.,
     Araya, S., Ohta, K. et al., 1992, {\it Memoirs of the Faculty
     of Science, Kyoto University,
     Series A of Physics, Astrophysics, Geophysics and Chemistry},
     {\bf 38}, 167

Osaki, Y., 1989, {\it PASJ}, {\bf 41}, 1005

Usher, P. D., 1981, {\it ApJ Suppl.}, {\bf 46}, 117

Vanmunster, T., 1995, {\it Cataclysmic Variables Circular}, No.\,47

Vanmunster, T., Howell, S. B., 1995, in ``Outburst Activity Data on Selected
    Cataclysmic Variables"

Vogt, N., Bateson, F. M., 1982, {\it Astron. Astrophys. Suppl.}, {\bf 48}, 383
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