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\IBVShead{5291}{14 June 2002}

\IBVStitletl{Optical monitoring of GM S\lowercase{gr} and discovery}{ of a Mira and a short-period pulsator}

\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}
\SIMBADobj{GM Sgr}
\IBVSkey{photometry}

\begintext

This Note is the third 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 a Mira
and a short-period pulsating star.  We also give the light curves of
GM Sgr, a previously known Mira (Orosz 2000, Kato~et~al. 2001).\\
\\
The reader is referred to our first Note (Gieles et al.\ 2002) for a
complete discussion of the telescopes and data reduction techniques
used. 
\begin{center}
\centerline{Table 1. Photometric data.}
\vskip 3mm
\begin{tabular}{ccccrccc}
\hline
\multicolumn{2}{c}{Coordinates (J2000)}	& $V$		& average 	&period		&$T_{0}$ 		& ID	\\ 
RA         & D				&		&  $V\!-\!I$	& (days)	& (HJD 2\,450\,000+)	& 	\\ \hline
18:19:36.7 & $-$25:25:53.1		&12.0\,$-$\,16.8	& 4.2 		&$\sim$208		& 2015			& \ldots \\
18:19:21.5 & $-$25:25:37.6		&13.1\,$-$\,18.2	& 3.6 	 	&212		& 2010			& GM Sgr  \\
18:19:11.0 & $-$25:23:20.3		&18.3\,$-$\,17.4	& 1.5  		&0.163		& 2015.025		& \dots  \\
\hline
\end{tabular}
\end{center}
\vskip 3mm

We found three stars that seem to be pulsating stars with well-defined
periods.  Only one of the three stars has been previously identified.
YALO J181921.5-252538 is Luyten's variable GM Sgr (Luyten 1927), which
in the past has been confused with V4641 Sgr. This star was
spectroscopically classified as a Mira star by Orosz (2000). Kato et
al.\ (2001) derived a photometric period of 212 days, in agreement
with what we find here.\\
\\
Table 1 gives an overview of the photometric information of
the three sources identified, including coordinates, magnitude and color 
ranges and ephemeris information ($T_{0}$ is the epoch of minimum light). 
Figs. 1 and 2 show light curves for
each source in different bands. Figs. 3 and 4 show the finding charts
for each source.  As in our previous Notes, 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).\\
\\
YALO J181936.7-252553 is a Mira very similar to GM Sgr (i.e. they both have
nearly the same period and $V$-band amplitude). YALO J181911.0-252320 is 
most probably a pulsating star of the
$\delta$ Scuti type.


\vspace*{1cm}

\IBVSfig{10cm}{5291-f1.eps}{Top: light curves of YALO J181936.7-252553 in $V$ and $I$. 
Bottom: light curves of YALO J181921.5-252538 = GM Sgr in the $V$ and $I$ band.}

\vspace*{-0.5cm}

\IBVSfig{4.7cm}{5291-f2.eps}{Folded light curves of YALO J181911.0-252320. 
Left: $V$-band. Right: $I$-band.}

\newpage

\vspace*{3cm}

\IBVSfig{16cm}{5291-f3.eps}{V-band finding charts of \\ YALO
J181936.7-252553 (left column) and GM Sgr = YALO J181921.5-252538
(right column). The top row shows single frames in which both sources
are at maximum brightness. The second row shows single frames in which
both sources at minimum brightness.}

\IBVSefigure{5291-f3a.eps}
\IBVSefigure{5291-f3b.eps}

\newpage

\IBVSfig{7.8cm}{5291-f4.eps}{V-band finding chart of \\ YALO
J181911.0-252320. Source is in centre.}


\references

Bailyn, C.D., Depoy, D., Agostinho, R., Mendez, R., Espinoza, J., \&
 Gonzalez, D., 1999, {\it BAAS}, {\bf 31}, 1502 
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Gieles, M., Orosz, J.A., Hulleman, F., Brogt, E., Bailyn, C.D., \&
 Garcia, M.R., 2002, {\it IBVS}, 5274

Kato, T., Uemura, M., Matsumoto, K., Ishioka, R., \& Iwamatsu, H.,
 2001, {\it IBVS}, 5133
%%\BIBCODE{2001IBVS.5133....1K}

Luyten, W.J., 1927, {\it Harvard Observatory Bulletin}, {\bf 851}
\BIBCODE{1927BHarO.851.....L}

Orosz, J.A., 2000, {\it IBVS}, 4921
%%\BIBCODE{2000IBVS.4921....1O}

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}

\endreferences


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