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\IBVShead{5071}{8 May 2001}

\IBVStitle{On the Supercycle of SX LM\lowercase{i}}

\IBVSauth{Kato, Taichi}

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

\SIMBADobj{SX LMi}
\IBVStyp{ UGSU }
\IBVSkey{dwarf nova}

\begintext

SX LMi is an SU UMa-type dwarf nova, which was originally discovered
as a faint blue star in the north Galactic pole region (Iriarte and Chavira
1957; Sanduleak and Pesch 1984).  The nature of the object was finally
revealed by the secure detection of superhumps during its outburst
in 1994 December (Nogami et al.\ 1997).

Nogami et al.\ (1997) argued that SX LMi has a relatively peculiar
character based on their extensive research on relations between outburst
amplitudes and supercycle lengths of known SU UMa stars.  Their most
important conclusion is that SX LMi has a relatively small superoutburst
amplitude ($\sim 3\fmm8$) while the suggested supercycle has a fairly
normal value of 250~d.  SX LMi was thus proposed as a candidate
for the missing link between ER UMa stars (for a review, see Kato et al.\ 1999)
and the usual SU UMa-type dwarf novae.  Other suggested candidates include
HS Vir (Kato et al.\ 1998), NY Ser (Nogami et al.\ 1998), CI UMa (Nogami and
Kato 1997) and V503 Cyg (Harvey et al.\ 1995).  However, none of these objects
show perfectly intermediate outburst characteristics between ER UMa stars
and usual SU UMa-type dwarf novae, as discussed in Kato et al.\ (2000).
SX LMi and CI UMa were listed in Kato et al.\ (2000) as objects having
less regular superoutbursts.  SX LMi was also discussed by Nogami et al.\
(1997) as having a possibly anomalous disk viscosity parameter.
Since the historical superoutbursts of SX LMi (also cited in Nogami et al.\
(1997)) looked to have occurred less regularly, a more comprehensive
search for superoutbursts is indispensable to reveal the nature of
the object, and to test the arguments presented by Nogami et al.\ (1997)
and Kato et al.\ (2000).

Observations reported to VSNET (http://www.kusastro.kyoto-u.ac.jp/vsnet/)
were analyzed, which almost completely covered the object since 1995 November,
except solar conjunction periods.  These visual observations were done
using $V$-magnitude calibrated comparison stars, and their errors are
an order of 0.2--0.3 mag, which will not affect the following discussion.
Figure 1 shows the overall light curve from VSNET observations.
Arrows represent upper limits, but upper limits brighter than 14.5 mag are
omitted from this figure in order to avoid confusion.  The light curve
clearly shows relatively regular occurrence of bright outbursts reaching
13.0--13.5 mag.  These outbursts might be naturally considered as
superoutbursts based on their duration and brightness, but we have tried to
adopt more secure identifications based on superhump detection.  Recent CCD
observations by Iwamatsu et al.\ (2001, in preparation) has confirmed that
the 2001 January long outburst is a genuine superoutburst.  Together with
past observations described in Nogami et al.\ (1997), we can now safely
identify long and bright outbursts as superoutbursts.

\begin{table}
\caption{Superoutbursts of SX LMi}
\begin{center}
\begin{tabular}{cccrc}
\hline
JD start & peak magnitude & \multicolumn3c{duration (d)} \\
\hline
2450238  & 13.2 & & $> 2$ \\
2450545  & 13.2 & & 14 \\
2450819  & 13.2 & & $> 4$ \\
2451131  & 13.1 & & 11 \\
2451631  & 13.1 & & 11 \\
2451935  & 13.1 & & $> 4$ \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10cm}{5071-f1.ps}{Light curve of SX LMi}

By assuming one missed superoutburst between JD 2451131 and 2451631
(the object was not observed because of the solar conjunction), these
observations indicate that SX LMi relatively regularly showed superoutbursts
with a cycle length of 250--312 d.  The mean period of supercycle is 279 d,
which is slightly longer than one suggested by Nogami et al.\ (1997).
The present observation has confirmed previously unrecognized regular
occurrence of superoutbursts, which also strengthen the finding by Nogami
et al.\ (1997) that normal outbursts are relatively infrequent compared
to the relatively short supercycle length.  This seems to make a striking
difference from HS Vir (Kato et al.\ 1998) and NY Ser (Nogami et al.\ 1998),
and suggests a different origin of the SX LMi peculiarity.

\bigskip

The authors are grateful to VSNET members, especially to Gary Poyner,
Gene Hanson, Timo Kinnunen, Mike Simonsen and Eddy Muyllaert for
providing crucial observations.

\references

Harvey, D., Skillman, D. R., Patterson, J., Ringwald, F. A., 1995,
{\it PASP}, {\bf 107}, 551

Iriarte, B., Chavira, E., 1957, {\it Boletin de Los Observatorios Tonantzintla y Tacubaya}, {\bf 16}, 3

Iwamatsu, H., et al., 2001, in preparation

Kato, T., Nogami, D., Masuda, S., Baba, H., 1998, {\it PASP}, {\bf 110}, 1400

Kato, T., Nogami, D., Baba, H., Masuda, S., Matsumoto, K., Kunjaya, C.,
1999, {\it Disk Instabilities in Close Binary Systems}, p.\ 45, eds.\
S. Mineshige, J. C. Wheeler (Universal Academy Press, Tokyo)

Kato, T., Hanson, G., Poyner, G., Muyllaert, E., Reszelski, M.,
Dubovsky, P. A., 2000, {\it IBVS}, No.\ 4932

Nogami, D., Kato, T., 1997, {\it PASJ}, {\bf 49}, 109

Nogami, D., Masuda, S., Kato, T., 1997, {\it PASP}, {\bf 109}, 1114

Nogami, D., Kato, T., Baba, H., Masuda, S., 1998, {\it PASJ}, {\bf 50}, 1L

Sanduleak, N., Pesch, P., 1984, {\it ApJS}, {\bf 55}, 517

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