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\IBVShead{5274}{15 May 2002}
\IBVStitle{Ten new semi-regular variables 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}

\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{IRAS 18160-2529}


\begintext

We have conducted an extensive CCD monitoring program of the black hole
binary (SAX J1819.3-2525 = V4641~Sgr) (Orosz~et~al. 2001). The
goal of this study was to acquire accurate light curves in different
bands to get better constraints on the physical parameters of the
binary. The results will be presented elsewhere. As a byproduct of this
campaign, we report here the discovery of ten new semi-regular
variable stars in the field of V4641~Sgr.\\

Optical photometry was done with the YALO 1~m telescope at Cerro Tololo
Interamerican Observatory (CTIO) (Bailyn~et~al. 1999), the CTIO
0.9~m telescope, the Dutch 0.9~m telescope at the European Southern
Observatory (ESO), La Silla and the 1.0~m Jacobus Kapteyn Telescope
(JKT) at La Palma (Table 1).  The data were collected between July~2000 
and October~2001. The IRAF data reduction package was used to
apply the standard flat field and bias corrections to the images. For
each telescope we made deep master images for each band by aligning
the frames to common coordinate systems and coadding them.  DAOPHOT (Stetson 1987, Stetson 1992) 
was used to do PSF
fitting and to compute the instrumental magnitudes for each
star. Stetson's program DAOMASTER was used to cross identify the stars
in each data set and to construct time series photometry. Standard
stars from the Landolt catalogue (Landolt 1992) observed
with the JKT were used to place the magnitude scales on the standard
system. \\

\begin{center}
\centerline{Table 1. Overview of the available datasets.}
\vskip 3mm
\begin{tabular}{cccccc}
\hline
Observing run	& Start date	& End date	& Frames& Filters		& FOV 	\\ \hline
YALO            & 27-Mar-2001	& 15-Oct-2001	& 98	& $V$ and $I$		& $10\arcm \times 10\arcm$\\
CTIO 		& 21-Jul-2000 	& 26-Jul-2000	& 255	& $U,B,V,R$ and $I$	& $3\farcm3 \times 3\farcm3$\\
ESO	        & 06-Jun-2001 	& 28-Jun-2001	& 61	& $U,B,V,R$ and $I$	& $3\farcm8 \times 3\farcm8$\\
La Palma	& 08-Sep-2000 	& 14-Sep-2000	& 24	& $B,V,R$ and $I$ 	& $8\farcm8\arcm \times 2\farcm8$\\
\hline
\end{tabular}
\end{center}
\vskip 1cm

\newpage
\begin{center}
\centerline{Table 2. Photometric data.}
\vskip 3mm
\begin{tabular}{ccccc}
\hline%
Coordinates (J2000)  		& $V$		& average $V\!-\!I$  	&quasi period & ID \\ 
& [mag] & [mag] & [days]\\ \hline
18:19:16.8~$-$25:23:36.2		&16.6\,$-$\,17.4 	& 4.41   		&51			\\
18:19:22.0~$-$25:23:16.8 		&16.6\,$-$\,17.3	& 4.50   		&76			\\
18:19:10.6~$-$25:27:39.1		&15.0\,$-$\,16.0	& 3.45   		&63			& IRAS 18160-2529  \\
18:19:02.5~$-$25:30:19.4		&15.8\,$-$\,16.6	& 4.37   		&\ \dots			\\
18:19:03.0~$-$25:29:34.5		&14.5\,$-$\,15.1	& 3.17   		&\ \dots			\\
18:19:03.7~$-$25:26:30.9		&15.6\,$-$\,16.1	& 2.81   		&\ \dots			\\
18:19:06.5~$-$25:24:11.5		&17.8\,$-$\,18.7	& 5.21   		&\ \dots			\\
18:19:28.0~$-$25:30:13.5		&15.2\,$-$\,15.7	& 4.12   		&67			\\
18:19:02.2~$-$25:24:05.9		&15.4\,$-$\,16.0	& 3.84   		&94			\\
18:19:40.8~$-$25:27:13.1		&16.5\,$-$\,16.9	& 4.29			&69			\\
\hline
\end{tabular}
\end{center}
\vskip 3mm
We have light curves for about 15\,000 stars, most of which appear
only in the YALO images, since the YALO telescope has the largest
field of view ($2048 \times 2048$ pixels with a pixel size of
$0\farcs3 \times 0\farcs3$, although the effective
field of view of the aligned images was about $9\arcm \times
9\arcm$). We searched for variables using two simple
techniques. First we computed the standard deviation $\sigma$ of the
light curves and made plots of $\sigma$ vs. the mean magnitudes.
Second, we computed Lomb-Scargle periodogram
(Lomb 1976, Scargle 1982) for each star and
sorted the stars according the highest L-S power. Once possible
variables were flagged (either large $\sigma$ or L-S power or both),
the light curves were visually inspected and additional periodicity
searches were done with the IRAF task `pdm', an implementation of the
phase dispersion technique of Stellingwerf
(Stellingwerf 1978). Our sensitivity to large amplitude
variables is high and we believe we have found $\gtrsim$ 95\% of the
bright ($V \gtrsim$ 17\mm) large amplitude ($\gtrsim$ 0.5 mag)
variables.\\

We found many variables. We report here a set of ten stars that seem
to form a homogeneous group, see Figs.~\ref{fig:lc1} and \ref{fig:lc2}
for light curves and Figs.~\ref{fig:first-fc}-\ref{fig:last-fc} for
charts. Table 2 gives the positions, magnitude range and average $V\!-\!I$
colours of the ten stars. An astrometric solution for the YALO master
image was found using stars from the USNOA2 catalogue and the
estimated 1$\sigma$ errors are $\lesssim$0\farcs5 in each
direction.  The names of the stars are based on their coordinates in
equinox 2000 and are given the prefix YALO.  For example the first
star in Table 2 at RA=18:19:16.8, DEC=$-$25:23:36.2 has the name YALO
J181916.8-252336. Only one star has a plausible identification in
SIMBAD: YALO~J181910.6-252739 is probably IRAS 18160-2529.\\ 

One of the characteristics of all ten stars is that they are
relatively red: average $V\!-\!I$ colours are between 2\fmm8 and 5\fmm2. The
extinction in the direction of V4641 Sgr is relatively low ($E(B-V)=0.25$, 
Orosz~et~al. 2001), so the red colours are not due to
interstellar dust. The stars show slow and quasi-periodic variations
with time scales between about 51 and 94 days and amplitudes in $V$
of $\sim$1 magnitude. The red colours, the relatively large amplitude
and the slow variability are characteristic of cool Asymptotic Giant
Branch (AGB) stars (Sterken and Jaschek 1996, Kerschbaum~et~al. 2001).


\newpage

\IBVSfig{20cm}{5274-f1.eps}{Light curves for five variables in $V$ (left panel) and $I$ (right panel).}\label{fig:lc1}
\vskip 1cm
\newpage
\IBVSfig{20cm}{5274-f2.eps}{Light curves for five variables in $V$ (left panel) and $I$ (right panel).}\label{fig:lc2}
\vskip 1cm
\newpage


\IBVS2fig{7.8cm}{5274-f3.eps}{$V$-band finding chart of \\ YALO J181916.8-252336. Source is in centre.\label{fig:first-fc}}{5274-f4.eps}{$V$-band finding chart of \\ YALO J181922.0-252317. Source is in centre.}

\IBVS2fig{7.8cm}{5274-f5.eps}{$V$-band finding chart of \\ YALO J181910.6-252739. Source is in centre. Note: The first star to the south-east is a variable star.}{5274-f6.eps}{$V$-band finding chart of \\ YALO J181902.5-253019. Source is lower-right of centre.}

\IBVS2fig{7.8cm}{5274-f7.eps}{$V$-band finding chart of \\ YALO J181903.0-252935. Source is right of centre.}{5274-f8.eps}{$V$-band finding chart of \\ YALO J181903.7-252631. Source is right of centre.}

\IBVS2fig{7.8cm}{5274-f9.eps}{$V$-band finding chart of \\ YALO J181906.5-252412. Source is in centre.}{5274-f10.eps}{$V$-band finding chart of \\ YALO J181928.0-253014. Source is below centre.}


\clearpage
\newpage



\IBVS2fig{7.8cm}{5274-f11.eps}{$V$-band finding chart of \\ YALO J181902.2-252406. Source is right of centre.}{5274-f12.eps}{$V$-band finding chart of \\ YALO J181940.8-252713. Source is left of centre.}\label{fig:last-fc}



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\endreferences


\end{document}

