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\IBVShead{4658}{5 January 1999}

\IBVStitle{NEW ECLIPSING BINARY HII\,706 IN THE PLEIADES CLUSTER}

\IBVSauth{S.-L. KIM$^1$, E. RODR\'IGUEZ$^2$}

\IBVSinst{Korea Astronomy Observatory, Taejon, 305-348, Korea}
\IBVSinst{Instituto de Astrof\'{\i}sica de Andaluc\'{\i}a, P.O. Box 3004, E-18080 Granada, Spain}

\begintext

\vfill

We present observational results of a newly discovered eclipsing binary
star HII\,706 (RA$_{2000} = 3\hr 45\mm 35\fsec 7$,
DEC$_{2000} = 24\deg 30\arcm 3\arcs$; V${} =11\fmm 85$, B$-$V${} =0\fmm
61$) in the central region of the Pleiades cluster. The light variations
of HII\,706 were detected accidentally during a multi-site observation
campaign (Rodr\'{\i}guez 1998) for four $\gamma$ Dor type variable
star candidates in the Pleiades cluster.

Time-series CCD photometry was performed over four nights from December
5 to 11, 1998.  The observations were done with a SITe 2048 CCD camera
attached to the 1.8m telescope at Bohyunsan Optical Astronomy
Observatory\,(BOAO). The field of view of the CCD image is
$11\farcm6\times11\farcm6$ at the f/8 Cassegrain focus of the
telescope.

Using the IRAF/CCDRED package, we processed CCD images to correct
overscan regions, trim unreliable subsections, subtract bias frames and
correct flat field images.  Instrumental magnitudes for HII\,706 and
comparison stars were obtained via the aperture photometry routine in
IRAF/DAOPHOT package (Massey \& Davis 1992).  Because seeing was
variable and typically about 2.0 arcsec during the observing runs, the
aperture size was chosen to be 18 pixels\,(6.12 arcsec) in diameter.
We applied classical two-star differential photometry to get
differential magnitudes.  Brightness of two comparison stars (V${}=11\fmm
26$, B$-$V${} =0\fmm 81$ for C1${}\equiv{}$HII\,746; V${}=13\fmm 84$,
B$-$V${}=0\fmm 89$ for C2${}\equiv{}$HII\,650, from our observation)
near HII\,706 was monitored to check light variations of HII\,706 (see
Figure 1).

We could not find any previous reports about light variations of
HII\,706 (Kholopov {\it et al.}\ 1985--1988, Durlevich {\it et
al.}\ 1996).  However, we detected obvious brightness change of the
star from our data.  Light curves of HII\,706 are shown in Figure 2 and
its differential magnitudes, $\Delta{\rm V}={\rm V}_{\rm HII\,706}-{\rm
V}_{\rm HII\,746}$, are listed in Table 1. Total V amplitude is about
0\fmm45 (11\fmm 85--12\fmm 30) and the period is estimated to be about
0.574 day (epoch at primary minimum is JD 2451157.06). Because the
light curves of phase from 0.0 to 0.5 and phase from 0.5 to 1.0 look
nearly the same, the star could be classified as a pulsating variable
star with a period of 0.287 day (a half of the above value). However,
considering the short period, large amplitude and peculiar light curves
(broad maximum and sharp minimum), we can rule out the possibility that
it belongs to pulsating stars such as $\delta$ Sct stars, $\gamma$ Dor
stars and RR Lyrae stars, {\it etc.} The light curves are similar to that
of an W UMa type eclipsing binary star (Hoffmeister {\it et al.}\
1985).

In Figure 3, we show the position of HII\,706 in color--magnitude
diagram of the Pleiades cluster. Magnitudes and colors in the BDA data
base (Mermilliod 1992) were used. From this, we can deduce that
HII\,706 may be a non-member star; it is about 1\fmm 5 fainter than
the main sequence stars in the cluster with the same color.

\vfill \eject

\IBVSfig{9.0cm}{4658-f1.ps}{Observed CCD frame
($11\farcm6\times11\farcm6$) of HII\,706 in the central region of the
Pleiades cluster. Two comparison stars (HII\,746, HII\,650) are denoted
by their Hertzsprung (1947) number}

\IBVSfig{9.0cm}{4658-f2.ps}{ Light variations of HII\,706. Magnitude
differences between the two comparison stars are also plotted in the lower
panel. Data points obtained over four nights are differently marked for
each observation night}

\vfill\eject

\IBVSfig{12cm}{4658-f3.ps}{Position of HII\,706 in the
color--magnitude diagram of the Pleiades cluster.  We use magnitudes
and colors in the BDA data base (Mermilliod 1992). HII\,706 (star
symbol) is about 1\fmm 5 fainter than main sequence stars in the
cluster with the same color}

\begin{table}
\caption{Differential magnitudes of HII\,706 ($\Delta{\rm V}={\rm V}_{\rm HII\,706}-{\rm V}_{\rm HII\,746}$)}
\begin{center}
\begin{tabular}{rc|rc|rc|rc}
\hline
\multicolumn{1}{c}{HJD} & $\Delta$V & \multicolumn{1}{c}{HJD} & $\Delta$V & \multicolumn{1}{c}{HJD} & $\Delta$V & \multicolumn{1}{c}{HJD} & $\Delta$V \\
\multicolumn{1}{l}{2451000 +} & & \multicolumn{1}{l}{2451000 +} & &  \multicolumn{1}{l}{2451000 +} & & \multicolumn{1}{l}{2451000 +} & \\
\hline 
153.07418 & 0.839 & 156.16336 & 0.844 & 157.14637 & 0.664 & 159.01099 & 0.726 \\ 
153.07561 & 0.830 & 156.22908 & 0.915 & 157.14786 & 0.668 & 159.08791 & 0.964 \\
153.16392 & 0.596 & 156.23059 & 0.903 & 157.23028 & 0.591 & 159.09087 & 0.963 \\
153.16559 & 0.596 & 156.27667 & 0.720 & 157.23173 & 0.597 & 159.19068 & 0.640 \\
155.97877 & 0.690 & 156.27814 & 0.713 & 157.27667 & 0.688 & 159.19222 & 0.626 \\
155.98024 & 0.684 & 156.96551 & 0.595 & 157.27818 & 0.692 & 159.26448 & 0.623 \\
156.05291 & 0.584 & 156.96715 & 0.595 & 158.94350 & 0.571 & 159.26600 & 0.638 \\
156.05430 & 0.594 & 157.05015 & 0.964 & 158.94504 & 0.569 &           &       \\
156.16183 & 0.836 & 157.05184 & 0.977 & 159.00652 & 0.711 &           &       \\
\hline
\end{tabular}
\end{center}
\end{table}

\vfill\eject

\references

Durlevich, O.V., Kazarovets, E.V., Kholopov, P.N., Kireeva, N.N.,
Samus, N.N., Tsvet\-kova, T.M., 1996, {\it The new computer-readable
version 1.1 of the General Catalogue of Variable Stars, fourth edition,
Vol.\,IV. The cross-identification tables}

Hertzsprung, E., 1947, Ann.\ Sterrewacht Leiden, 19, 3

Hoffmeister, C., Richter, G., Wenzel, W., 1985, in {\it Variable stars}, p.\,202

Kholopov, P.N., Samus, N.N., Frolov, M.S., Goranskij, V.P., Gorynya,
N.A., Kireeva, N.N., Kukarkina, N.P., Kurochkin, N.E., Medvedeva,
G.I., Perova, N.B., Shugarov, S.Yu., 1985-1988, {\it General Catalogue
of Variable Stars}, 4th Edition (Moscow: Nauka Publishing House)

Massey, P., Davis, L.E., 1992, {\it A User's Guide to Stellar CCD
photometry with IRAF}

Mermilliod, J.-C., 1992, {\it Open cluster database, BDA (Version I.5)}, data updated by the end of 1997

Rodr\'{\i}guez, E., 1998, {\it private communication}

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