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\IBVShead{5124}{18 June 2001}

\IBVStitle{Outbursts of CG D\lowercase{ra}}

\IBVSauth{Kato, Taichi$^1$; Nogami, Daisaku$^{2,1}$}

\IBVSinst{Dept.\ of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp}

\IBVSinst{Hida Observatory, Kyoto University, Gifu 506-1314, Japan,
e-mail: nogami@kwasan.kyoto-u.ac.jp}

\SIMBADobj{CG Dra}
\IBVStyp{ UGSS }
\IBVSkey{dwarf nova, photometry}

\begintext

CG Dra is a faint dwarf nova discovered by Hoffmeister (1966).  He
reported frequent occurrence of outbursts and a small outburst
amplitude.  Bruch et al.\ (1987) observed this object on eight nights
and found one outburst.  Cannon Smith et al.\ (1997) obtained spectra
and detected the feature of a K-type secondary.  Bruch et al.\ (1997)
found spectral type of K$5\pm2$ for the secondary.  Bruch et
al.\ (1997) also detected variations in the observed radial velocities
of Balmer emission lines.  From these variations, they suggested a
possible orbital period of 0\fday1893 or 0\fday2343.  However, they
argued that the spectral type of K$5\pm2$ corresponds to a longer
orbital period of $\sim0\fday27$. Bruch et al.\ (1997) also found
inconsistencies between the radial velocities of emission lines and the
absorption features, which they attributed to the secondary.  These
inconsistencies suggest that either the canonical model is wrong, or
the object is a peculiar system.

The observations were done on six nights between 1996 May 6 and July 29,
using a CCD camera (Thomson TH~7882, $576 \times 384$ pixels,
on-chip $2 \times 2$ binning adopted) attached to the Cassegrain focus of
the 60-cm reflector (focal length${} = 4.8$ m) at Ouda Station, Kyoto University
(Ohtani et al.\ 1992).  An interference filter was used which had been
designed to reproduce the Johnson $V$ band.  The exposure time was
60--120 s depending on the brightness of the object.  The frames were
first corrected for standard de-biasing and flat fielding, and were then
processed by a microcomputer-based PSF photometry package developed by
one of the authors (TK)\@.  The magnitudes were determined relative to
GSC 3920.1216 (GSC magnitude 13.12), whose constancy during the run was
confirmed using the check star GSC 3920.954 (GSC magnitude 14.67).
Table 1 lists the log of observations, together with nightly averaged
magnitudes.  The overall light curve is shown in Figure 1.

Two outbursts were observed, both on their fading stages.  The high
frequency of outbursts is also inferred from this observation.
The outburst cycle length is shorter than 82 d.  Both outbursts faded
very slowly.  The first outburst showed a linear decline at a rate of
0.14 mag\,d$^{-1}$.  The second outburst showed a slightly varying
decline rate, and its nominal average was 0.31 mag\,d$^{-1}$.  Although
the data points are few to accurately determine the typical decline rate
of this object, the values on the both occasions are remarkably smaller
than decline rates in other dwarf novae (cf.\ Warner 1995).
This is consistent with the spectroscopic evidence that CG Dra shows
a large contribution from the secondary, suggesting a long orbital period.
Since DX And (Kato and Nogami 2001), having an orbital period of 0\fday4405,
showed a rate of decline of 0.35 mag\,d$^{-1}$,
Bailey's relation (cf.\ Szkody and Mattei 1984; Warner 1995) suggests
an even longer period for CG Dra.  From the photometric point of view,
we support the longer orbital period inferred from the spectroscopic
classification of the secondary.  The apparent periodicity in the
radial velocity variation, as already argued by Bruch et al.\ (1997),
seems to more reflect something other than the orbital motion itself.

\begin{table}
\normalsize
\begin{center}
\caption{Nightly averaged magnitudes of CG Dra}
\begin{tabular}{cccc|cccc}
\hline
mid-JD$^a$ & mean mag$^b$ & error$^c$ & $N^d$ & mid-JD & mean mag & error & $N$ \\
\hline
50210.181 & 2.532 & 0.059 & 5 & 50292.042 & 3.064 & 0.087 & 4 \\
50213.266 & 2.917 & 0.059 & 3 & 50293.130 & 3.538 & 0.058 & 5 \\
50218.291 & 3.634 & 0.140 & 3 & 50294.125 & 3.699 & 0.111 & 5 \\
\hline
\end{tabular}

\begin{tabular}{l@{ }ll@{ }l}
$^a$ & JD${}-{}$2400000 & $^b$ & Magnitude relative to GSC 3920.1216 \\
$^c$ & Standard error of nightly average & $^d$ & Number of frames\\
\end{tabular}
\end{center}
\end{table}

\IBVSfig{9cm}{5124-f1.ps}{Overall light curve of CG Dra}

\vskip-24pt
\references

Hoffmeister, C., 1966, {\it AN}, {\bf 289}, 139

Bruch, A., Fischer, F.-J., Wilmsen, U., 1987, {\it A\&AS}, {\bf 70},
481

Bruch, A., Schimpke, T., Kochsiek, A., 1997, {\it A\&A}, {\bf 325}, 601

Cannon Smith, R., Sarna, M. J., Catal\'{a}n, M. S., Jones, D. H. P.,
1997, {\it MNRAS}, {\bf 287}, 271

Kato, T., Nogami, D., 2001, {\it IBVS}, No.\ 5098

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

Szkody, P., Mattei, J. A., 1984, {\it PASP}, {\bf 96}, 988

Warner, B., 1995, Cataclysmic Variable Stars (Cambridge Univ.\ Press.,
Cambridge)

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