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\IBVShead{4613}{23 July 1998}

\IBVStitle{On the Orbital Period of EG~Cancri}

\IBVSauth{K.~Matsumoto$^1$, T.~Kato$^1$, K.~Ayani$^2$, T.~Kawabata$^2$}

\IBVSinst{Department of Astronomy, Faculty of Science, Kyoto University, Kyoto 606-8502, Japan,\\
e-mail: katsura@kusastro.kyoto-u.ac.jp, tkato@kusastro.kyoto-u.ac.jp}
\IBVSinst{Bisei Astronomical Observatory, 1723-70 Ohkura, Bisei, Okayama 714-1411, Japan,\\ 
e-mail: ayani@bao.go.jp, kawabata@bao.go.jp}

\SIMBADobj{EG Cnc}
\IBVStyp{ dwarf nova }
\IBVSkey{photometry}

\begintext

The dwarf nova EG~Cnc was discovered by Huruhata (1983).  He reported
the detection of an outburst in 1977 November and no sign of any other
outburst in his photographic archives taken between 1977 and 1983.
McNaught (1986) indicated that the object in quiescence was 18.6~mag
(red) and 18.0~mag (blue) on the Palomar Sky Survey prints.  He pointed
out that characteristics of the object, which are 1) the outburst
amplitude over 6~mag, 2) the decline rate of 2~mag per 20~days, and 3)
its blue color, were similar to those of WZ~Sge and its group.

Although there had been no confirmed outburst in EG~Cnc since the 1977
outburst, the object underwent a rare outburst on 1996 November 30
(Figure~1).  The object was $m_{\rm vis}~\simeq~12$ at the first
detection by Schmeer (1996).  The outburst was a superoutburst, and it
is known that the wonderful six repeated mini-outbursts appeared in the
latter half of the superoutburst (Kato et al.\ 1998).  In the first
half of the superoutburst, the object showed two types of superhumps,
namely 1) a double-peaked early superhump and 2) a following ``common''
superhump (Matsumoto et al.\ 1998).  The early superhump showed a
slightly shorter modulation period of 0\fday 0582 than that of the
superhump of 0\fday 06036, later appeared.  Matsumoto et al.\ (1998)
suspected the former period is the orbital period of the binary system,
by an application of the discussion in the 1996 superoutburst of AL~Com
(Kato et al.\ 1996), and also discussed the similarities between
EG~Cnc and the WZ~Sge type dwarf novae.  The WZ~Sge type dwarf novae
have some peculiar characteristics such as 1) very long supercycle
exceeding years, 2) showing only superoutbursts without normal
outbursts, and 3) large amplitudes of the superoutbursts over 6~mag.
They are also known to occupy the shortest orbital-period region among
all CVs.  Confirmation of the orbital period of EG~Cnc is crucial
because it will provide an additional, independent support to
the claimed universal presence of ``early superhumps'' in WZ Sge
stars.  We report here our observations of EG~Cnc in quiescence to
detect orbital modulations.


The CCD photometric observations were made on four nights between 1997
December 27 and 1998 January 2, using the 101-cm reflector at Bisei
Astronomical Observatory and the 60-cm reflector at Ouda Station, Kyoto
University.  The observations at Bisei and Ouda were carried out with
Johnson $V$-band interference filters and liquid-nitrogen cooled CCD
cameras of TEK~1024T ($1242 \times 1152$ pixels) and Thomson TH~7882
($576 \times 384$ pixels) at the Cassegrain foci, respectively.  For
more details of the instruments, see Ayani et al.\  (1996) for Bisei,
and Ohtani et al.\  (1992) for Ouda.  A summary of the observations
is given in Table~1.

\begin{table}[t]
\caption{The log of the observations}
\begin{center}
\begin{tabular}{lcccc}
\hline
{Date} & {HJD$-$2400000} & {No.\ of frames} & {Error (mag)} & {Observatory} \\
\hline
1997 Dec 27 & 50810 & 46 & 0.06 & Bisei \\
1997 Dec 27 & 50810 & 55 & 0.15 & Ouda \\
1997 Dec 28 & 50811 & 22 & 0.20 & Ouda \\
1997 Dec 31 & 50814 & 67 & 0.20 & Ouda \\
1998 Jan 2  & 50816 & 50 & 0.18 & Ouda \\
\hline
\end{tabular}
\end{center}
\end{table}

The frames obtained at Bisei were reduced in a standard way using the
IRAF (distributed by the National Optical Astronomy Observatories,
U.S.A.), and the instrumental stellar magnitudes were measured with the
aperture photometry routine (APPHOT) in the IRAF.  The reductions of
the Ouda frames were carried out with a microcomputer-based automatic
PSF photometry package developed by one of the authors (T.K.).  We
determined the relative magnitudes of the object using a local
comparison star of 12.82~mag (V) lying $\sim 4'$ west of the object.

\IBVSfig{8cm}{4613-f1.eps}{Overall light curve of the 1996
superoutburst of EG~Cnc.  The filled circles and squares respectively
represent the CCD observations at Osaka Kyoiku University and Ouda
station (Matsumoto et al.\ 1998, Kato et al.\ 1998).  The open circles
represent the reports by VSNET members.}

\vfil\eject

\IBVSfig{8cm}{4613-f2.eps}{$\chi^{2}$ tests for periods of the orbital hump}
\IBVSfig{8cm}{4613-f3.eps}{Folded light curve}

In order to find a period of modulations in quiescence of EG~Cnc,
sine-curve fittings were applied to the obtained light curve with trial
periods between 0.02--0.09~d.  Figure~2 presents the results of
$\chi^{2}$ for the each trial period of the fittings.  The confidence
levels are represented by the dashed-lines with their value in the
figure.  The $\chi$ squares indicate a minimum at around 0.05--0.06~d,
and there are four possible periods of 0.0528, 0.0551, 0.0557, and
0.0589~day over 70$\%$ confidence level.  By knowing the superhump
period of 0\fday 06036 during the 1996 superoutburst of this object
(Matsumoto et al.\ 1998; Patterson et al.\ 1998), the period of
0\fday 0557 corresponds to an 8.4$\%$ fractional superhump excess, ${\rm
(P_{SH}-P_{orb})/P_{orb}}$.  This excess value is far beyond the usual
relation (excess versus ${\rm P_{SH}}$) among SU UMa-type dwarf novae
(eg.\ see Table~1 in Nogami et al.\ 1997), and each period less
than 0\fday 0557 will not be a good candidate for the orbital period.
Therefore we consider that the 0\fday 0589 with a 2.5$\%$ fractional
superhump excess is the most likely period which reflects the orbital
period of the system from the present data.  The present result
consequently supports that the period of the early superhump in the
1996 superoutburst corresponded to the orbital period.


Figure~3 is the light curve of the all data folded by the
0\fday 0589 period and binned to 0.1 phases.  The dashed-line in the figure
represents the best-fit sine curve based on the result of the
$\chi^{2}$ tests.  According to the fitted curve, the times of
photometric maxima are expressed by the following ephemeris:
\begin{equation} {\rm HJD~(Maximum)} = 2450810.0912 + 0.0589 \times E,
\end{equation} and the orbital modulation of EG~Cnc in quiescence has
an amplitude of $\sim$~0.06~mag.

The observation at Bisei Astronomical Observatory was made as a part of
the 3rd observational program in 1997 of the observatory (K.M.).





\references

Ayani, K., Ohshima, O., Shimizu, M., Kogure, T., 1996, in Proceedings
of Third East-Asian Meeting on Astronomy {\it ``Ground-based Astronomy
in Asia''} (ed.\ N.~Kaifu, National Astronomical Observatory, Japan),
421

Huruhata, M., 1983, {\it IBVS}, No.\,2401

Kato, T., Nogami, D., Baba, H., Matsumoto, K., Arimoto, J., Tanabe, K.,
Ishikawa, K., 1996, {\it PASJ}, {\bf 48}, L21

Kato, T., Nogami, D., Matsumoto, K., Baba, H., 1998, {\it ApJ},
submitted

Matsumoto, K., Nogami, D., Kato, T., Baba, H., 1998, {\it PASJ}, {\bf 50}, 405

McNaught, R.H., 1986, {\it IBVS}, No.\,2926

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

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

Patterson, J., Kemp, J., Skillman, D.R., Harvey, D.A., Shafter, A.W.,
Vanmunster, T., Jensen, L., Fried, R., Kiyota, S., Thorstensen, J.R.,
Taylor, C.J., 1998, {\it PASP}, submitted

Schmeer, P., 1996, vsnet-alert {\bf 599} (available on \\
http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-alert/msg00599.html)

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