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\IBVShead{5117}{12 June 2001}

\IBVStitle{RX C\lowercase{ha}: New Long-period SU UM\lowercase{a}-type Dwarf Nova}

\IBVSauth{Kato, Taichi$^1$; Garradd, Gordon$^2$; Stubbings, Rod$^3$; Pearce, Andrew$^4$;\\ \indent Nelson, Peter$^5$}

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

\IBVSinst{PO Box 157, NSW 2340, Australia,
e-mail: loomberah@ozemail.com.au} % GAR

\IBVSinst{19 Greenland Drive, Drouin 3818, Victoria, Australia,
e-mail: stubbo@qedsystems.com.au} % Stu

\IBVSinst{32 Monash Ave, Nedlands, WA 6009, Australia,
e-mail: Andrew.Pearce@worley.com.au} % Pea

\IBVSinst{RMB 2493, Ellinbank 3820,Australia,
e-mail: pnelson@dcsi.net.au} % NEL

\SIMBADobj{RX Cha}
\IBVStyp{ UGSU }
\IBVSkey{dwarf novae -- photometry -- astrometry}

\begintext

RX Cha is a little studied, faint dwarf nova.  Vogt and Bateson (1982)
provided a likely identification with a faint blue star on SRC plates.
Bruch et al.\ (1992) provided astrometry of the likely quiescent counterpart.
Two attempts have been made to spectroscopically identify the object
in quiescence (Zwitter and Munari 1996; Munari and Zwitter 1998), but
no spectroscopic information was obtained due to the faintness of the
object.  Zwitter and Munari (1996) gave an upper limit of $V =20.5$ for
the quiescent counterpart.  The large outburst amplitude ($>6$ mag)
made RX Cha as a good candidate for an SU UMa-type dwarf nova.
The object has been regularly monitored by visual observers, and
several outbursts have been recorded.

Visual observations were done by using 32-cm (R.S.), 40-cm (A.P.) and
32-cm (P.N.) reflectors.  All observations were done using photoelectrically
calibrated $V$-magnitude comparison stars.  The typical error of visual
estimates was less than 0\fmm2 mag, which does not affect the following
discussion.  During the 1998 September outburst, time-resolved CCD photometry
and astrometry were performed by one of the authors (G.G.), with an
unfiltered AP-7 CCD attached to a 45-cm reflector.  The exposure time
was 60 s.  A total of 216 CCD frames were taken between BJD 2451073.077
and 2451073.232.  Table 1 lists the observed outbursts since 1998 January.

\IBVSfig{10cm}{5117-f1.ps}{Light curve of RX Cha on 1998 September 16}

\IBVSfig{10cm}{5117-f2.ps}{Periodogram of RX Cha}

\vskip-22pt
Figure 1 shows the CCD light curve on 1998 September 16.  The magnitudes
are given relative to GSC 9405.598 (Tycho-2 magnitude $V=11.63\pm0.13$,
$B-V=+1.15\pm0.31$), whose constancy during the run was confirmed using the
check star GSC 9405.1400 (Tycho-2 magnitude $V=12.11\pm0.18$).  The light
curve shows two superhumps with an amplitude of 0.15--0.20 mag.
The period analysis was done using the Phase Dispersion Minimization (PDM)
method (Stellingwerf 1978).  The resultant theta diagram is shown in
Figure 2.  The best superhump period is determined as $0.0839\pm0.0020$ d.
However, the apparently changing amplitude of superhumps may have slightly
affected the result of the analysis.

\begin{table}
\begin{center}
\caption{Outbursts of RX Cha}
\begin{tabular}{ccc}
\hline
JD start & peak magnitude & duration (d) \\
\hline
2450831 & 14.5 & --- \\
2451066 & 14.4 & $> 8$ \\
2451544 & 14.4 & $> 8$ \\
2451982 & 14.3 & $> 9$ \\
\hline
\end{tabular}
\end{center}
\end{table}

The CCD observation during the 1998 September outburst has confirmed
that RX Cha is an SU UMa-type dwarf nova.  The resultant superhump period
of 0\fday084 makes RX Cha an SU UMa-type dwarf nova with a long orbital
period ($P_{\rm orb}$).  Astrometry using 80 GSC stars has yield the
following accurate position (mean residual 0\farcs4): 10\hr36\mm26\fsec33,
$-80\deg 02\arcm 48\farcs2$ (J2000.0).  This confirms the identification
by Vogt and Bateson (1982), and the inferred large outburst amplitude of
$>6$ mag.

Some of the long orbital period SU UMa-type dwarf novae, such as YZ Cnc
and SS UMi, tend to have a high outburst frequency.  The low number of
detected outbursts (Table 1) clearly suggests that outbursts are relatively
rare in this system.  All detected outbursts, except the first one, have
long durations and are identified as superoutbursts.  The first one was
not well covered by observations, but the brightness may also suggest
a superoutburst.  The supercycle is thus $\sim460$ d, if the first outburst
is a normal one, or its half, $\sim230$ d, if the first outburst
is a superoutburst.  The lack of detections of definite normal outbursts
between well-observed superoutbursts may have been a result of the faintness
of the object, but is more likely to directly reflect the low number of
normal outbursts.  Such a low number ratio of
(normal outbursts)/(superoutbursts) is a common property in
SU UMa-type dwarf novae with low outburst frequencies.  However, such
systems are known to be rare among long $P_{\rm orb}$ systems.  Only a few
systems are known to show similar properties: EF Peg (Matsumoto et al.,
in preparation), V725 Aql (Uemura et al.\ 2001) and DV UMa (e.g.\ Nogami
et al.\ 2001).  Since these systems play an important role in understanding
the evolution of dwarf novae, and the origin of mass-transfer, further
detailed observations of RX Cha are highly encouraged.

\bigskip

This work was done as a part of VSNET Collaboration
(http://www.kusastro.kyoto-u.ac.jp/vsnet/).

\references

Bruch, A., Meijer, J., Naumann, M., Schimpke, T., Ungruhe, R., Vogt, N.,
1992, {\it A\&AS}, {\bf 93}, 463

Matsumoto et al., in preparation

Munari, U., Zwitter, T., 1998, {\it A\&AS}, {\bf 128}, 277

Nogami, D., Kato, T., Baba, H., Novak, R., Lockley, J. J., Somers, M.,
2001, {\it MNRAS}, {\bf 322}, 79

Stellingwerf, R. F., 1978, {\it ApJ}, {\bf 224}, 953

Uemura, M., Kato, T., Pavlenko, E., Baklanov, A., Pietz, J., 2001,
{\it PASJ}, in press

Vogt, N., Bateson, F. M., 1982, {\it A\&AS}, {\bf 48}, 383

Zwitter, T., Munari, U., 1996, {\it A\&AS}, {\bf 117}, 449

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