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\IBVShead{5070}{8 May 2001}

\IBVStitle{Outburst Photometry of DK C\lowercase{as}}

\IBVSauth{Kato, Taichi; Uemura, Makoto}

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

\SIMBADobj{DK Cas}
\IBVStyp{ UGSS }
\IBVSkey{dwarf nova}

\begintext

DK Cas is a poorly studied dwarf nova which was discovered by
Hoffmeister (1943).  The fourth edition of the General Catalogue of
Variable Stars gives a range of 15.3--19.5~p.  Little has been known
regarding its nature and outburst characteristics.  However, there have
been a few evidences that outbursts of this object are relatively
rare.  Bruch et al.\ (1987) observed this object on 17 nights and found
no outbursts.  A vigorous search for outbursts since 1995 by visual and
CCD observers (mainly by T. Vanmunster and G. Poyner), contributed to
VSNET (http://www.kusastro.kyoto-u.ac.jp/vsnet/) had yielded no
positive detections until the detection by P. Schmeer on 1999 November
27 (Schmeer 1999).  Since the low frequency of outbursts as well as a
relatively large outburst amplitude makes DK Cas a good candidate for
an SU UMa-type dwarf nova, we started time-resolved CCD photometry.

The CCD observations were done using an unfiltered ST-7 camera attached
to the Meade 25-cm Schmidt--Cassegrain telescope.  The exposure time was
30 s.  The images were dark-subtracted, flat-fielded, and analyzed
using the Java$^{\rm TM}$-based PSF photometry package developed by one
of the authors (TK)\@.  The magnitudes were determined relative to BD
+56\deg 35 = GSC 3661.1642, whose Tycho-2 magnitude is
$V=10.02\pm0.03$ and $B-V=+0.28\pm0.04$.  The constancy of comparison
star during the run was confirmed by comparison with GSC 3661.1306.
The log of observations together with nightly average magnitudes is
given in Table~1.  The light curve drawn from these data is presented
in Figure~1.

\begin{table}
\caption{Nightly averaged magnitudes of DK Cas}
\begin{center}
\begin{tabular}{ccccr}
\hline
JD start$^a$ & JD end$^a$ & mean mag$^b$ & error$^c$ & $N^d$ \\
\hline
51509.992 & 51510.024 & 5.075 & 0.049 & 37 \\
51511.002 & 51511.185 & 4.978 & 0.004 & 415 \\
51512.056 & 51512.176 & 4.986 & 0.005 & 302 \\
51516.187 & 51516.292 & 5.243 & 0.015 & 262 \\
51520.162 & 51520.165 & 5.803 & 0.061 & 10 \\
51521.147 & 51521.150 & 5.995 & 0.140 & 10 \\
51521.981 & 51521.984 & 6.309 & 0.090 & 10 \\
51522.975 & 51522.979 & 6.603 & 0.122 & 10 \\
51523.965 & 51523.969 & 6.923 & 0.098 & 8 \\
\hline
\multicolumn5l{\normalsize
\begin{tabular}{c@{ }l}
$^a$ & JD${}-{}$2400000 \\
$^b$ & Magnitude relative to BD +56\deg 35 \\
$^c$ & Standard error of nightly average \\
$^d$ & Number of frames \\
\end{tabular}} \\
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10cm}{5070-f1.ps}{Light curve of DK Cas}

\vskip-11pt
DK Cas reached a maximum (unfiltered CCD magnitude 14.86, roughly
corresponding to an $R_c$ magnitude assuming the usual color close to
$B-V=0.0$ for outbursting dwarf novae) within two days of the outburst
detection.  The object stayed at maximum for three days and started
fading slowly.  Time-resolved CCD photometry during the outburst
plateau showed only slow variation with small random fluctuations, and no
clear indication of periodic modulations (Figure~2).  In addition to
the presence of a short plateau at maximum, followed by a slow fade,
the absence of clear superhump modulations is sufficient to rule out
the object as being an SU UMa-type dwarf nova.  The suggested
classification of an SS Cyg-type star (UGSS) is supported by this
observation.

\IBVSfig{10cm}{5070-f2.ps}{Time-series photometry of DK Cas. Each point
represents an average of adjacent five frames.  The magnitudes are
arbitrarily shifted for clarity}

\vskip-11pt
The later part of the fading from outburst is characterized by a linear
fade at a rate of $0.30\pm0.01$ mag\,d$^{-1}$, which is relatively slow
among SS Cyg-type dwarf novae.  Based on the calibration by Szkody and
Mattei (1984) of Bailey's relation, the orbital period of DK Cas is
expected to be longer than 5 hours, and is most likely longer than
that of a long-period system DX And (10.6 hours, Drew et al.\ 1993).
Although recent spectroscopic observation by Liu and Hu (2000) was not
able to detect a feature of the secondary, the above data suggest that
DK Cas belongs to a rare class of long-period dwarf novae with a low
outburst frequency (i.e.\ low mass-transfer rate), a further spectroscopic
search for the secondary and accurate determination of the orbital period
are highly encouraged.

\bigskip

The authors are grateful to VSNET members for providing dense
observations covering years, and P. Schmeer for promptly and publicly
notifying the outburst.  Part of this work is supported by a Research
Fellowship of the Japan Society for the Promotion of Science for Young
Scientists (MU).

\references

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

Drew, J. E., Jones, D. H. P., Woods, J. A., 1993, {\it MNRAS}, {\bf 260}, 803

Hoffmeister, C., 1943, {\it Publ.\ Berlin-Babelsberg Univ.\ Obs.}, 28

Liu, Wu, Hu, J. Y., 2000, {\it ApJS}, {\bf 128}, 387

Schmeer, P., 1999, {\it VSNET alert circulation}, No.\ 3745 (available from\\
   http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/alert3000/msg00745.html)

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

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