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\IBVShead{5122}{18 June 2001}

\IBVStitletl{Observation of Superhumps in IR G\lowercase{em}}

\IBVSauth{Kato, Taichi}

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

\SIMBADobj{IR Gem}
\IBVStyp{ UGSU }
\IBVSkey{dwarf nova, photometry}

\begintext

IR Gem is a well-known SU UMa-type dwarf nova.  However, little
observation of superhumps has been reported since the identification as
an SU UMa-type dwarf nova (Szkody et al.\ 1984).  We observed this star
during its 1991 March superoutburst.

The observations were done on two successive nights, 1991 March 18 and
19, using a CCD camera (Thomson TH~7882, $576 \times 384$ pixels,
on-chip $3 \times 3$ 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 $I_{\rm c}$ band.  The
exposure time was 10 s.  The frames were first corrected for standard
de-biasing and flat fielding, and were then processed by a
microcomputer-based aperture photometry package developed by the
author.  The magnitudes of the object were determined relative to GSC
1905.753 (GSC magnitude 11.07), whose constancy during the run was
confirmed using the check star USNO A1.0-1125.04589035.  Barycentric
corrections to observed times were applied before the following
analysis.  Table 1 lists the log of observations, together with nightly
averaged magnitudes.

Figure 1 shows the resultant light curve.  Superhumps are prominently
seen.  After removing the trend of decline, we applied Phase Dispersion
Minimization (PDM) method (Stellingwerf 1978).  The resultant theta
diagram is shown in Figure 2.  The result generally confirms the superhump
period of 0\fday07076 reported by Szkody et al.\ (1984).  The best period
determined from our data is $0.07094\pm0.00006$ d, which is slightly
longer than that by Szkody et al.\ (1984).  By taking the orbital period
of 0\fday0684 (Feinswog et al.\ 1988), the fractional superhump excess
is 3.7\%.  The most remarkable difference of superhumps from those
observed by Szkody et al.\ (1984) is the clear presence of secondary
superhumps, i.e.\ bump-like feature on the fading branch of superhumps.
The feature was markedly seen on 1991 Mar 18, but became less clear
on the subsequent night.  This feature was discussed by Udalski (1990)
on SU UMa itself.  Udalski (1990) proposed that this feature may arise
from a cooler component of the disk, but the nature is not still well
understood.  The appearance of secondary superhumps in $I_{\rm c}$ band
light curve may be consistent with Udalski's (1990) hypothesis.

\IBVSfig{10cm}{5122-f1.ps}{Light curve of the 1991 March superoutburst
of IR Gem}

\IBVSfig{10cm}{5122-f2.ps}{Periodogram of IR Gem}

\clearpage

\begin{table}
\begin{center}
\caption{Log of observations}
\begin{tabular}{ccccc}
\hline
start$^a$ & end$^a$ & mean mag$^b$ & error$^c$ & $N^d$ \\
\hline
48333.932 & 48334.084 & 2.234 & 0.002 & 760 \\
48334.936 & 48335.115 & 2.420 & 0.002 & 816 \\
\hline
\end{tabular}

\normalsize
\begin{tabular}{l@{ }l}
$^a$ & BJD${}-{}$2400000\\
$^b$ & Magnitude relative to GSC 1905.753\\
$^c$ & Standard error of nightly average\\
$^d$ & Number of frames\\
\end{tabular}
\end{center}
\end{table}

\references

Feinswog, L., Szkody, P., Garnavich, P., 1988, {\it AJ}, {\bf 96}, 1702

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

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

Szkody, P., Shafter, A. W., Cowley, A. P., 1984, {\it ApJ}, {\bf 282},
236

Udalski, A., 1990, {\it AJ}, {\bf 100}, 226

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