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\IBVShead{4414}{19 December 1996}

\title{Variable stars in the Globular Cluster NGC\,6681}

\begintext

    NGC\,6681  (M\,70, $C1840-323, l = 2\fdg 9, b =-12\fdg 5$)  
 is a variable poor Oo II cluster (II VP). With $[Fe/H]
=  -1.51$  it  is  one  of  several  clusters  which lie in the
metallicity  interval where alongside with II VP there coexist
I  VR  (Oo  I  variable  rich)  clusters.  The  cluster has an
absolute  magnitude  $M_{v_{o}}  = -7\fm05$, concentration class CC = V,
apparent  radius $r=3\farcm9$ (Kukarkin 1974) and limiting radius 
$r = 11\farcm0$ (Kukarkin \& Kireeva, 1979).

       The  cluster  was  first searched for variable stars by
Rosino (1962). In the investigated area he
discovered  five  variable  stars,  three  of  which are field
variables  with  a  distance $r > 11\farcm7$ from the cluster center.
The  other  two variables,  designated  V1  and  V3,  at $r = 2\farcm0$ and $9\farcm8$
respectively,  were  noted  by  Rosino as RR?, and at the time of
publication  of  the  Third  Catalogue  of  Variable  Stars in
Globular  Clusters  (Sawyer  Hogg  1973)  were  the only known
variables   assumed to be cluster members.

      A further search for variables was made by Liller (1983).
In the investigated field   she   discovered   18  variables  
including  the  five previously detected by Rosino. Three of 
the variables found by Liller  with  r  =  $0\farcm9$,  $5\farcm9$  
and $8\farcm3$ are within the limiting radius of the cluster. 
The estimated periods for four of them (excluding V4) enabled 
their classification as RR Lyrae variables. However according to 
the ``mean" magnitude two of  the latter V5 and V2 (renamed Rosino V3) 
at r = $8\farcm3$ and $9\farcm8$ were  found  to have low probability 
membership, thus limiting the  distance  of  the  three cluster 
variables at r = $5\farcm9$. In Liller's  table  1  there  is a 
misprint in the signs of the X coordinates   of   V3   and  V4,  
which  should  be $ - $ and $ + $ respectively.



   Details of the CCD observations, obtained  with  the 0.9 m Dutch 
telescope at ESO-La Silla,  methods  of reduction are
given  in  the paper by Brocato et al. (1996). The method of a search 
for as yet undiscovered variables was the same as in the companion paper  (Kadla et al., 1996).  The V and B magnitudes of the measured stars are based 
on 16 photoelectric standards (Landolt, 1992) within the magnitude intervals 
$12\fm52 < V < 15\fm91$ and $13
\fm02 < B < 16\fm13$ ($-0\fm24 < B-V < 1\fm91$).
CMD was obtained using for the mean V and B magnitudes from
3 V and 3 B consecutive exposures, the time difference between the 
mean V and B magnitudes being 25 minutes. 




 The stars in the instability strip of the resulting CMD 
diagram are shown in Figure~1. The available photometric data (23 exposures -
15 V and 8 B) 
permitted to confirm the variability of nine stars (including 
the aforementioned two known RR Lyr variables discovered by 
Rosino and Liller). Data for these variables (numbered 1 - 12, 
open circles) and other stars in the instability strip are given 
in Table 1 and Figure 2. Six stars (Nos. 13 - 18, triangles) are probably  
variables but need further confirmation and three (Nos. 19 - 21, 
$V> 16\fm36$) are probably field variables.
There are three stars (Nos. 22 - 24, asterisks) in the instability 
strip which did not show any sign of variability in our data.
If the variable V4 without a determined period at r = $5\farcm9$, 
found by Liller, is included there are at present ten known RR Lyrae 
stars belonging to the cluster. 

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\centerline{2}
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\begin{center}
Figure 1. The CMD for stars in an area $3\farcm8$ $\times$ $3\farcm8$ 
centered on the cluster. Concerning notation of stars in the instability strip see  the text
\end{center}
\vskip 5mm

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\centerline{\epsffile{4414-f2.ps}}
\begin{center}
Figure 2. Chart of the cluster showing the position stars  in Table 1
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\centerline{3}
\vskip 1mm
\centerline{Table 1. Photometric data for variables}
\vskip 1mm                                       
\begin{center}
\begin{tabular}{rrrccrrrcc}
\hline\noalign{\smallskip}
  $N$  &   $X$ &  $Y$ &  $V$ &  $B-V$ &  $N$  &   $X$ &  $Y$ &  $V$ &  $B-V$\\
     & (arcsec) & (arcsec) &   &  &     & (arcsec) & (arcsec) &  &  \\
%\noalign{\smallskip}
\hline\noalign{\smallskip}

   1 &56.4 & $-$105.1& 15.58&  0.45&  13&  $-$19.3&$-$12.0&16.06&  0.59\\
   3 &$-$9.2&$-$53.2&15.91&  0.52&  14&   $-$9.4& 2.5& 15.31&  0.14 \\
   6 &$-$12.9 &$-$30.0  &15.55&  0.33&  15&   $-$1.6&23.2& 15.41 &  0.35 \\
   7 & $-$9.3 & $-$1.2  &15.55&  0.41&  16&    1.7& 6.9& 16.10 &  0.56 \\
   8 & $-$8.2 &  0.4  &15.84&  0.24&  17&   8.7 &$-$19.1& 15.52&  0.29 \\
   9 &  2.1 & 20.3  &15.47&  0.23&  18&   37.1&$-$17.9& 15.81&  0.28 \\
  10 &  5.5 &  2.1  &15.21&  0.26&  19&  $-$19.7&  9.2& 16.36&  0.31 \\
  11 &  5.6 & $-$7.1  &16.16&  0.45&  20&  $-$10.9& $-$7.4& 16.54&  0.56 \\
  12 & 45.9 & 20.3  &15.43&  0.27&  21&   45.9& 40.0& 16.40&  0.67 \\
     &      &       &     &      &  22&    2.6&  4.3& 15.01&  0.12 \\
     &      &       &     &      &  23&   $-$4.0&$-$22.7& 15.56&  0.26 \\
     &      &       &     &      &  24&  $-$54.1& 34.8& 15.61&  0.25 \\
\hline

%\noalign{\smallskip}                 
%\hline\noalign{\smallskip}                               
\end{tabular}                        
\end{center} 
\vskip 3mm
 Financial support from Russian Fund for Basic Research
is gratefully acknowledged.
                    
\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.M.:
 1996, {\it A\&A}, {\bf 312}, 80

Kadla, Z.I., Gerashchenko, A.N. \& Malakhova, Yu.N., 1997, 
 {\it IBVS}, No.\,4413

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

Kukarkin, B.V. \& Kireeva, N.N.: 1979, {\it Astr. Zh.}, {\bf 56}, 465
[English transl.: {\it Soviet Astr.}, {\bf 23}, 261]

Landolt, A.U.: 1992, {\it AJ}, {\bf 104}, 340

Liller, M.: 1983, {\it AJ}, {\bf 88}, 1463

Rosino, L.: 1962, {\it Mem. Soc. Astron. Ital.}, {\bf 33}, 351, No.\,5
% Original: Rosino, L.: 1962, {\it Mem. Soc. Astron. Ital.}, No.\,5, {\bf 33}, 351
% Corrected: Tue Jul 29 09:39:06 MET DST 2003

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

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