\documentstyle[twoside, epsf]{article}

\input ibvs2.sty
\begin{document}

\IBVShead{4634}{25 September 1998}

\IBVStitle{Searching the Open Cluster NGC\,6939 for Variable Stars}

\IBVSauth{R.M. ROBB and R.D. CARDINAL}

\IBVSinsto{Climenhaga Observatory, Dept.\ of Physics and Astronomy, 
University of Victoria, Victoria, BC, Canada, V8W 3P6, 
Internet: robb@uvic.ca}

\IBVSabs{ A high precision photometric search has found six variable stars}
\IBVSabs{in the open cluster NGC 6939.}

\SIMBADobj{NGC 6939}
\IBVStyp{ Open Cluster }
\IBVSkey{ stars: variable}
\IBVSkey{ binaries: eclipsing}

\begintext

Small radial velocity variations in red giants were discovered by Walker
et al. (1989) suggesting that corresponding photometric variations 
might be detectable.
Edmonds and Gilliland (1996) discovered photometric variations in the
red giant stars  in the globular cluster 47 Tucanae, 
using the Hubble Space Telescope in the ultraviolet.
We observed NGC 6939 in an attempt to observe similar variations
in an open cluster in the visible part of the spectrum. 

Our sixteen nights of observations were made using the 0.5 meter 
telescope, Cousins R filter and CCD camera of
the University of Victoria (Robb and Honkanen 1992).  This is 
an automated system, which will observe a single field all night,
keeping a star on the same pixels.
Each frame was bias 
subtracted and flat fielded using   
IRAF.\footnote{IRAF is distributed by National Optical
Astronomy Observatories, which is operated by the Association of
Universities for Research in Astronomy, Inc., under contract to the
National Science Foundation} 
To isolate merged images the point-spread-function-fitting routines 
in DAOPHOT 
(Stetson et al. 1990) were used to find the magnitudes.  An ensemble was
formed
from the brightnesses of the brightest stars and each star was compared
with the ensemble to form a differential magnitude, $\Delta R$
in the sense of the star minus the ensemble.   

The red giant stars measured in common with
Mermilliod et al. (1994) are listed in Table 1 with 
our star identification numbers, Kustner's (1923) identification numbers,
the V and B$-$V found by Mermilliod et al. (1994), the $\Delta R$
and the night to night standard deviation.


\begin{table} [h]
\caption{Brightness, color, $\Delta R$, and its precision for the red
giants.}
\begin{center} 
\begin{tabular}{ccc ccc | ccc ccc}
\hline
Id & K Id & V & B$-$V & $\Delta R$ &  StD.&
Id & K Id & V & B$-$V & $\Delta R$ &  StD.\\
\hline
s1 & 212 & 12.03& 1.69& 2.867 &0.002&s3 & 133 & 12.27& 1.51& 3.187&0.005\\
s4 & 121 & 12.53& 1.54& 3.448 &0.003&s6 & 135 & 12.58& 1.11& 3.740&0.002\\
s8 & 190 & 12.73& 1.36& 3.840 &0.005&s9 & 182 & 12.92& 1.37& 3.835&0.003\\
s10 & 053 & 12.86& 1.30& 3.894&0.004&s11 & 134 & 12.95& 1.29& 4.020&0.004\\
s12 & 230 & 12.90& 1.32& 4.051&0.005&s13 & 170 & 13.13& 1.34&4.041&0.003\\
s15 & 145 & 13.05& 1.31& 4.086&0.002&s18 & 214 & 13.00& 1.27&4.185&0.006\\
s19 & 130 & 13.10& 1.25& 4.184&0.003&s20 & 294 & 13.19& 1.34& 4.209&0.012\\
s24 & 58 & 13.32& 0.89& 4.568&0.002&s26 & 279 & 13.73& 0.96& 4.918 &0.010\\
\hline
\end{tabular}
\end{center}
\end{table}



Brightness variations during a night were measured by the standard 
deviation of the differential magnitudes of that night, which
ranged from 0\fmm 004 for bright stars on
a good night to 0\fmm 100 for the faint stars on poor nights.
The standard deviation of the sixteen nightly means is a measure 
of the night to night variations. 
The high precision of these data can be seen from the standard deviation of
$\Delta R$ for the bright stars.
The fainter stars have the expected larger standard deviation.
Plots of the standard deviation versus brightness were made and all
outliers were checked for variability.  

To find the variable stars the differential magnitude of each star was
plotted against the time of day.  For each star the sixteen nights were 
plotted  in a four by four array with the brightness and time scales 
chosen to be the same for all nights.  At a glance we could then 
see both the variations during a night and also from night to night.  We 
checked all 215 stars in this manner and surprisingly found no ambiguity
in whether or not a star was variable.  

\IBVSfig{9cm}{4634-f1.ps}{Finder chart labeled with our identification
numbers}

A finder chart based on our frames is shown in figure 1 with the variable
stars marked with our id numbers.  Table 2 gives our  
star identification numbers, Kustner's (1923) id numbers, 
coordinates (J2000) and magnitudes measured using
the Hubble Space Telescope Guide Star Catalog (Jenkner et al., 1990),
the period, the epoch with the uncertainty in the final digit in
parentheses and the type of epoch.

\begin{table}
\caption{Variable stars discovered in the field of NGC 6939.}
\begin{center}
\begin{tabular}{llcccllc}
\hline
Id. No. &Kustner& R.A. & Dec. &   & Period & Epoch & Epoch \\
  &      Id     &J2000&J2000& Mag.&[days] & Helio JD & Type\\
\hline

s11&K134&$20\hr31\mm27\sec$&$+60\deg$38\arcm18\arcs&13.3&7.4(6)&2450657.5(7)
& max\\ 
s28&K125&$20\hr31\mm25\sec$&$+60\deg$40\arcm18\arcs&14.2&1.30(2)&2450655.75(9)
& max\\
s33&K80&$20\hr31\mm13\sec$&$+60\deg$40\arcm31\arcs&14.4&10.5(20)&2450654.4(9)
& max\\
s63&K95&$20\hr31\mm19\sec$&$+60\deg$38\arcm10\arcs&15.1&4.954(3)&2450653.8079(5)
& min\\
s98&K147&$20\hr31\mm29\sec$&$+60\deg$39\arcm35\arcs&15.6&3.598(3)&2450654.0244(8)
& min\\
s154&-&$20\hr31\mm14\sec$&$+60\deg$36\arcm47\arcs&16.6&0.3550(6)&2450656.432(5)
& min\\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{9cm}{4634-f2.ps}{R band light curves of the variable stars in 
NGC 6939 plotted according to phase.}

Each variable star's differential magnitudes were fit to a sine curve 
of various periods and the $\chi^2$ was used to estimate the best
period.  Light curves were then plotted at these periods and the aliases
and multiples were checked. 
All the periods are very secure except that
of star s11 where the period finding program
also found an alias of 1.165 days that was nearly as likely.


The data were binned by JD four points to a bin.  The mean magnitudes of
each star are plotted against phase in Figure 2 using the period and epoch 
given in Table 2.
The stars s11, s28 and s33 all lie in the red giant part of the
HR diagram but s33 is probably a field star (Cannon and Lloyd 1969).
Of the 16 red giants measured by Mermilliod et al. (1994) and ourselves,
we find that one is variable (s11), 13 have a standard deviation of
0\fmm005 or less from night to night and the other two were at the 
extreme corner of the CCD and have a standard deviation of about 0\fmm01. 
This is in contrast to Jorissen et al. (1997), who propose that all
late type stars are variable. 
S11 was observed for radial velocity by Milone (1994) who found it to have
the largest scatter among those stars not considered variable.

Obviously from the light curves the stars s63 and s98 are eclipsing
binaries.  Using the method of Kwee and van Woerden (1956), 
heliocentric Julian times of secondary minimum were found to be
2450695.9169(5) for star s63 and 2450691.8036(8) for star s98. From the
light curve of s154 we classify it as a contact binary.  From our period
and the relations of Rucinski (1997)  and assuming a reddening of
$E_{(B-V)}=0.43$ (Mermilliod et al. 1994), this W UMa star's apparent
brightness
is consistent with it being a member of the cluster.

These data show that the K giant variable stars can be observed from the
ground in the visible light and that not all red giant stars are variable
at the level of accuracy of this work.  
The two eclipsing binary stars are near the turnoff point of the cluster
and so are deserving of more observations especially radial velocity
measurements to find the masses of the stars.



\vskip 1cm
\references

Cannon, R.D. and Lloyd, C., 1969, {\it MNRAS}, {\bf144}, 449

Edmonds P. and Gilliland R., 1996, {\it ApJ}, {\bf464}, L157

Jenkner, H., Lasker, B., Sturch, C., McLean, B., Shara, M., Russell, J., 1990,
{\it AJ}, {\bf 99}, 2082

Jorissen, A., Mowlavi, N., Sterken, C., Manfroid, J., 1997, {\it A\&A}, 
{\bf 324}, 578

Kustner, F., 1923, Ver\"off. Univ. Sternw. Bonn, No.\,18

Kwee, K.K. and van Woerden, H., 1956, {\it BAN}, {\bf 12}, 327

Mermilliod, J.C., Huestamendia, G., del Rio, G., 1994, {\it A\&A}, {\bf106},
419

Milone, A.A.E., 1994, {\it PASP}, {\bf106}, 1085

Robb, R.M. and Honkanen, N.N., 1992, A.S.P. Conf. Ser., {\bf 28}, 105,
Automated Telescopes for Photometry and Imaging, ed. Adelman, Dukes and 
Adelman
%Original: Robb, R.M. and Honkanen, N.N., 1992, in A.S.P. Conf. Ser., {\bf38}, 
%Automated Telescopes for Photometry and Imaging, ed. Adelman, Dukes and 
%Adelman, 105
%Changed: Fri Sep 16 10:39:54 MEST 2005

Rucinski, S.M., 1997, {\it AJ}, {\bf 113}, 407

Stetson, P.B., Davis, L.E., Crabtree, D.R., 1990, A.S.P. Conf. Ser., 
{\bf 8}, 289, CCDs in Astronomy, ed. by Jacoby
%Changed: Thu Sep 29 14:01:51 MEST 2005

Walker, G.A.H., Yang, S., Campbell, B., Irwin, A.W., 1989, {\it ApJ},
{\bf343}, L21

\end{document}


