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\IBVShead{5289}{7 June 2002}

\IBVStitle{Discovery of four close binary stars in Sagittarius}

\IBVSauth{GIELES, M.$^1$; OROSZ, J.A.$^1$; HULLEMAN, F.$^1$; BROGT, E.$^2$; BAILYN, C.D.$^3$; GARCIA, M.R.$^4$}

\IBVSinst{Astronomical Institute, Utrecht University, P.O. Box 80\,000,
3508 TA Utrecht, The Netherlands,\\ e-mail: gieles@astro.uu.nl, orosz@astro.uu.nl, hulleman@astro.uu.nl}

\IBVSinst{Kapteyn Institute, University of Groningen, P.O. Box 800, 
9700 AV Groningen, The Netherlands,\\ e-mail: brogt@astro.rug.nl}

\IBVSinst{Department of Astronomy, Yale University, P.O. Box 208\,101,
New Haven, CT 06510-8101, United States, e-mail: bailyn@astro.yale.edu}

\IBVSinst{Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA\,02138, United States, e-mail: garcia@head-cfa.harvard.edu}

\SIMBADobj{V4641 Sgr}

\IBVSkey{photometry}

\begintext


This Note is the second in a series of Notes where we report new
variable stars discovered during our extensive CCD monitoring program
of the black hole binary V4641~Sgr (SAX J1819.3-2525) (Orosz~et~al.,\ 2001). 
In this Note we report on the discovery of four
close binary stars (i.e. binaries where the radius of one or both stars
is a substantial fraction of the orbital separation).\\
\\
The reader is referred to our first Note (Gieles~et~al., 2002) for a
complete discussion of the telescopes and data reduction techniques
used.  Briefly, CCD observations in the $U$, $B$, $V$, $R$, and $I$
filters were collected between July 2000 and October 2001 using 4
different 1m class telescopes.  We have the light curves of about 
15\,000 stars in a 9\arcm $\times$ 9\arcm field.  We searched for
variables by comparing the standard deviation $\sigma$ of the light
curves and by computing the maximum power in a Lomb-Scargle
periodogram (Lomb~1976, Scargle~1982) for each star.  These two
techniques are well suited for finding either large amplitude
variables (these have large values of $\sigma$) or smaller amplitude
periodic variables (these have large L-S power). Although it is
difficult to be quantitative about our completeness for finding
variables, we believe our completeness is reasonably high.

\begin{center}
\centerline{Table~1. Photometric data.}
\vskip 3mm
\begin{tabular}{cccccc}
\hline
Coordinates (J2000)  		& $V$		& average $V\!-\!I$  	&period (days)	& Type of variable\\ \hline
18:19:24.8~$-$25:24:56.6		&17.5\,$-$\,18.1	& 0.82 			&0.3668		& ellipsoidal 	\\
18:19:00.9~$-$25:24:12.1		&14.6\,$-$\,14.8	& 1.22  		&77.0		& ellipsoidal	\\
18:19:07.8~$-$25:27:15.0		&15.8\,$-$\,15.2	& 0.85 	 		&0.464		& W UMa  \\
18:19:24.4~$-$25:25:53.5		&16.4\,$-$\,18.1	& 0.75  		&0.8273		& Algol	\\
\hline
\end{tabular}
\end{center}
\vskip 3mm

We found four stars which are close binary stars. Table 1 gives an
overview of the photometric information of the four sources identified.
  Figs. 1 to 4 show the light curves for each source in
different bands. Figs. 5 to 8 show the finding charts for each
source. As in our previous Note, the names of the stars are based on
their coordinates in equinox 2000 and are given the prefix YALO since
most of the variables were discovered with the data of the YALO
telescope (Bailyn~et~al. 1999). None of these sources appear in SIMBAD.\\
\\
The light curves of YALO J181924.8-252457 and YALO J181900.9-252412
give a strong suggestion that these sources are close binaries where
at least one star (nearly) fills its Roche lobe. Since YALO
J181900.9-252412 has a relatively long period of 77 days, the star
which dominates the light is probably a K giant as it is in the well-known 
binary 5~Ceti (Eaton and Barden 1988).\\
\\
YALO~J181907.8-252715 is most probably a W UMa type binary (Moss and
Whelan,~1970) in which both star overfill their Roche lobes. The two
stars probably have nearly the same temperature since the minima are
of nearly equal depth.\\
\\
YALO J181924.4-252554 is most probably an Algol variable.



\references

Bailyn, C.D., Depoy, D., Agostinho, R., Mendez, R., Espinoza, J., \&
 Gonzalez, D., 1999, {\it BAAS}, {\bf 31}, 1502 
\BIBCODE{1999AAS...195.8706B}

Eaton, J.A., \&  Barden, S.C., 1988, {\it Acta Astronomica}, {\bf 38}, 353
%%\BIBCODE{1988AcA....38..353E}

Gieles, M., Orosz, J.A., Hulleman, F., Brogt, E., Bailyn, C.D., \&
 Garcia, M.R., 2002, {\it IBVS}, {\bf 5274}

Lomb, N.R., 1976, {\it Ap\&SS}, {\bf 39}, 447
%%\BIBCODE{1976Ap&SS..39..447L}

Moss, D.L., \& Whelan, A.J., 1970, {\it MNRAS}, {\bf 149}, 147
%%\BIBCODE{1970MNRAS.149..147M}

Orosz, J.A., Kuulkers, E., van der Klis, M., McClintock, J.~E., 
Garcia, M.~R., Callanan, P.J., Bailyn, C.~D., Jain, R.~K., \& Remillard, R.~A.,
2001, {\it ApJ}, {\bf 555}, 489
%%\BIBCODE{2001ApJ...555..489O}

Scargle, J.D., 1982, {\it ApJ}, {\bf 263}, 835
%%\BIBCODE{1982ApJ...263..835S}



\endreferences



\newpage

\IBVSfig{20cm}{5289-f1.eps}{Folded light curves of YALO J181924.8-252457. Left: unbinned data. 
         Right: data binned to 40 phase bins. $T_0$ is HJD 2\,450\,000+. Note: the $U$-band
        values are instrumental magnitudes.}
\vskip 1cm

\IBVSfig{9.05cm}{5289-f2.eps}{Folded light curves of YALO J181900.9-252412. Left: unbinned data. 
         Right: data binned to 30 phase bins. $T_0$ is HJD 2\,450\,000+}
\vskip 1cm
 
\IBVSfig{9.05cm}{5289-f3.eps}{Folded light curves of YALO J181907.8-252715. Left: unbinned data. 
         Right: data binned to 30 phase bins. $T_0$ is HJD 2\,450\,000+}
\vskip 1cm

\IBVSfig{20cm}{5289-f4.eps}{Folded light curves of YALO J181924.4-252554. Left: unbinned data. 
         Right: data binned to 40 phase bins. Note: the $U$-band
        values are instrumental magnitudes. $T_0$ is HJD 2\,450\,000+}
\vskip 1cm

\newpage
\pagebreak



\IBVS2fig{7.8cm}{5289-f5.eps}{V-band finding chart of \\ YALO J181924.8-252457. Source is in centre. Note: $\sim 3^{\prime\prime}$ to the west two known variable stars are indicated: V4641 Sgr which is a black hole binary and GM Sgr which is a Mira variable.}{5289-f6.eps}{V-band finding chart of \\ YALO J181900.9-252412. Source is right of centre.}

\IBVS2fig{7.8cm}{5289-f7.eps}{V-band finding chart of \\ YALO J181907.8-252715. Source is in centre.}{5289-f8.eps}{V-band finding chart of \\ YALO J181924.4-252554. Source is in centre.}



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