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\IBVShead{4413}{19 December 1996}


\title{Variable stars in the Globular Cluster M72}

\begintext

      NGC\,6981 (M72), l$ = 35\fdg 2, b = -33\fdg 7$, is a variable-rich Oo IA cluster.    The cluster is classified as CC IX (concentration
class) and according to Kukarkin (1974) has a radius $R=2\farcm94$.
   The list of 42 stars in the Third Catalogue of Variable
stars (Sawyer-Hogg 1973) contains 28 RR Lyrae variables
with known periods, 11 with no period determinations, 2 non-variables 
and one red variable (V42).
The four variables (V2, V27, V35, V39) detected at larger
distances $3\farcm 6 \leq R \leq 5\farcm 2$, all have a positive X coordinate.
Periods have been determined for the first three and hence
there are 25 known RR Lyr variables with $R\leq2\farcm5$. A comparison of the
position of the stars on the reproduced plates with the accompanying
lists of Shapley (1920), Shapley and Ritchie (1920) and Sawyer-Hogg (1953) indicates that the X coordinates of V29 and V41 should  have an opposite sign, i.e. minus and plus
correspondingly. These errors have not been corrected in catalogue of
Sawyer-Hogg (1973).

     During the past 40 years no further search had been made
for variable stars in this cluster and no periods were
determined for 13 (V6, V19,V22, V26, V30, V33, V34, V36 - V41).
In order to check the variability of the 11 stars with
unknown periods and search for as yet undetected variables
the method proposed and applied for M3 (Kadla \& 
Gerashchenko, 1982) was used. It is based
on an analysis of a color--magnitude diagram obtained from
measurements of two plates (or CCD) taken ``simultaneously".
A variable is thus at identical phase and the RR Lyrae stars
are located in a definite strip. By indicating the possible
variables the diagram considerably narrows down the number
of stars which need further investigation.

     We had at our disposal a pair of the necessary CCD (B,V)
exposures obtained with the 90 cm Dutch telescope at La Silla.
   Details   on  the  observations,  methods  of
reduction are given  in  the paper by Brocato et al. (1996). The  field 
includes 33 stars with $R < 2\farcm5$  in the variable star list
(Sawyer-Hogg, 1973). Photoelectric standards obtained by Dickens (1972) were used
to transform the instrumental magnitudes. The resulting V -- (B$-$V)
diagram (Figure 1) includes 239 stars in the magnitude range 
$15\fm 50 < V < 18\fm 00$, the known variable stars being denoted by an asterisk.

     The positions of V6, V19 and V33 on the color--magnitude diagram
indicate that they belong to the GB. Of the four stars (V13, V22, V26 
and V34) located in the vicinity of the RHB stars only V13 has a known 
period, V22 is listed as a non-variable and the last two are most
probably RHB stars. It was difficult  to identify the 5 variables
with $R\leq0\farcm4$ found by Sawyer-Hogg (1953). The reproduced
plate of the cluster does not have the necessary quality and no
known variables are marked, although according to the listed coordinates
V40 is located close to the known variable V13.

     The coordinates, V and B$-$V of the 9 suspected variable 
stars in the RR Lyr variability strip are given in Table 1.
Their positions were determined using as a reference frame the 
coordinates system given in the catalogue of Sawyer-Hogg (1973).

\vfill\eject

\centerline{2}
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\begin{center}
Figure 1. The color--magnitude diagram for the globular cluster NGC\,6981.
The known RR Lyrae stars are denoted by asterisks,
the stars from Table 1 (S) - circles , (R) - pluses.
\end{center}
\vskip .5cm

\begin{center}
Table 1. Positions and photometric data for suspected variables (S) and
for possible variable stars  located at the intersection with the RHB(R)
\end{center}
\vskip 2mm

\begin{center}
\begin{tabular}{crrccrrcrrcc}
\hline\noalign{\smallskip}
 $N$ & $X$ & $Y$ & $V$ & $B-V$ & & & $N$ & $X$ & $Y$ & $V$ & $B-V$\\
     & (arcsec) & (arcsec) &  & &  &  &     & (arcsec) & (arcsec) &  &   \\
\noalign{\smallskip}              
\hline\noalign{\smallskip}        
S1 & $-$44.9 & $-$36.5& 16.60 & 0.27& & & R1 &$-$68.2&$-$102.6 & 16.92 & 0.54 \\
S2 & $-$15.9 &  $-$9.5 & 16.83 & 0.38 & & & R2&$-$34.9&$-$55.4 & 16.99 & 0.53 \\
S3 & $-$10.0 &  $-$4.5 & 16.61 & 0.26 & & & R3 & $-$23.4&$-$9.2 & 17.01 & 0.56 \\
S4 & $-$0.4 & $-$11.9 & 17.20 & 0.54 & & & R4 &   3.7 &   6.7 & 16.97 & 0.53 \\
 S5 &   0.4 &   2.7 & 16.29 & 0.34 & & & R5 &  18.2 &  26.8 & 17.01 & 0.52 \\
 S6 &   5.4 & $-$17.5 & 16.22 & 0.50 & & &R6&  32.3 & $-$12.4 & 16.93 & 0.53 \\
 S7 &   9.6 &  $-$2.4 & 16.36 & 0.26 & & &R7&  50.9 & $-$44.4 & 16.96 & 0.53 \\
 S8 &  13.6 &  $-$9.0 & 16.72 & 0.27 & & &    &       &       &       &      \\
 S9 &  24.9 &  $-$9.0 & 16.75 & 0.46 & & &    &       &       &       &      \\
\noalign{\smallskip}                 
\hline                               
\end{tabular}                        
\end{center}
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\centerline{3}
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Figure 2. Chart of the cluster. The notations V, S and R preceding 
the star number refer to known and suspected (Table 1) variables.
\end{center}

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\centerline{\epsffile{4413-f3.ps}}
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Figure 3. The central part of NGC\,6981. The notation are the same as 
in Figure 2.
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\centerline{4}
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     As the observed RR Lyrae instability strip slightly intersects the
     RHB there  is  a  possibility  that  some  of the latter stars are
     variable. The coordinates, V and B$-$V of the stars which
should  be  checked  are listed in Table 1. 
The maps of NGC\,6981 with
known and suspected variable stars are shown in Figure 2 and Figure 3 (coordinates are in pixels, 1 pixel = 0\farcs44).
Almost all the suspected variables are located in the central part
of cluster. 

\vskip 1cm

\beginauthors  
\authorsrightcol
Z.I. KADLA 
A.N. GERASHCHENKO 
Yu.N. MALAKHOVA
Central Astronomical Observatory of 
the Russian Academy of Sciences at 
Pulkovo, 196140 Saint--Petersburg, 
Russia,
e-mail: kadla@pulkovo.spb.su
\endauthors

\references

Brocato E., Buonanno R., Malakhova Yu. \& Piersimoni A., 1996, 
{\it A\&Ap}, {\bf 312}, 80

Dickens R.J., 1972, {\it MNRAS}, {\bf 157}, 281

Kadla Z.I. Gerashchenko A.N., 1982, {\it Isv. Main A.O. of the Academy
 of Sci. of USSR (Pulkovo)}, {\bf 199}, 86

Kukarkin B.V., 1974, The General Catalogue of Globular Clusters
 of our Galaxies. Publishing House Nauka, Moscow.

Sawyer-Hogg H., 1953, {\it Comm. David Dunlap Obs.}, No.\,34

Sawyer-Hogg H., 1973, {\it Publ.David Dunlap Obs. 3}, No.\,6

Shapley H., 1920, {\it ApJ.}, {\bf 52}, 73, {\it Mt. Wilson Contr. No.\,190}
% Original: Shapley H., 1920, {\it Mt. Wilson Contr. No.\,190 ApJ.}, {\bf 52}, 73
% Corrected: Tue Jul 29 09:35:07 MET DST 2003

Shapley H., Ritchie M., 1920, {\it ApJ}, {\bf 52}, 232, {\it Mt. Wilson Contr. No.\,195}
% Original: Shapley H., Ritchie M., 1920, {\it Mt. Wilson Contr. No.\,195; ApJ}, {\bf 52}, 232
% Corrected: Tue Jul 29 09:35:07 MET DST 2003

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