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\IBVShead{4434}{5 February 1997}

\title{Variable stars in the Globular Cluster M12}

\begintext

      NGC\,6218 (M12, C1644-018, $l = 15\fdg 7$, $b = 26\fdg 3$)
is a variable-poor cluster. It is an intermediate metallicity cluster 
with concentration class  IX, apparent radius $r=7\farcm2$ (Kukarkin, 1974) 
and tidal radius $r = 13\farcm4$ (Webbink, 1985).

   The only variable found in this cluster by Sawyer-Hogg (1938) 
is a long-period Cepheid. Later Clement et al. (1988) confirmed her
result that it was a W Vir variable (number 1 in  Table 1) 
and discovered its period to be unstable, ranging from $15\fd 50$ to $15\fd 55$.
      
 In our investigation we study four fields with the common size  of
$4\farcm3 \times 4\farcm3$. One may find 
details of the observations and data reduction in the paper by Brocato et al. (1996).
Search for variable stars in the cluster was made in the same way as described
in our previous paper (Kadla et al., 1996).
From 18 $V$ and 20 $B$ frames we selected for our study 8 $B,V$ pairs with 
the time interval between $B$ and $V$ exposures shorter 
than 10 min. There are two stars (numbers 2 and 3 in Table 1) 
in the instability strip of the colour -- magnitude diagram (Figure 1) that can be considered as  
RR Lyrae variable candidates. Unfortunately the duration of observations was 
insufficient to confirm the variability of these stars. Data for suspected 
variables (coordinates, $V$ magnitudes and colour $B-V$) are listed in Table 1. 

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Figure 1. The color - magnitude diagram for the globular cluster M12,
suspected variables are marked by circles
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A finding chart for M12, suspected variables are denoted by circles
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\centerline{Table 1. Positions and photometric data for suspected variables}
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\begin{tabular}{rrrcc}
\hline
 $N$ & $X$\ \ \ & $Y$\ \ \ & $V$ & $B-V$\\
     & pixels & pixels &  &  \\
\hline
\\
1 &    366.00  &    96.52 & 12.02 & 0.90 \\
2 &   $-$116.63  &   558.41 & 14.85 & 0.57 \\
3 &    262.75  &   498.57 & 15.11 & 0.58 \\                                             \hline                               
\end{tabular}                        
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\beginauthors
\authorsrightcol
Yu.N. MALAKHOVA 
A.N. GERASHCHENKO
Z.I. KADLA
Central Astronomical Observatory of 
the Russian Academy of Sciences at
Pulkovo, 196140 Saint--Petersburg, 
Russia, e-mail: mal@pulkovo.spb.su
\endauthors
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\references

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

Clement Ch.M., Hogg H.S., Lee A., 1988, {\it AJ}, {\bf 96}, No.\,5, 1642

Kadla Z.I., Gerashchenko A.N. \& Malakhova Yu.N., 1996, {\it IBVS}, No.\,4413 

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

Sawyer-Hogg H., 1938, {\it Publ. Domin. Astroph. Obs.}, {\bf 7}, No.\,5

Webbink R.F., 1985, IAU Symposium, No\,113, 541,
    eds by J. Goodman \& P. Hut, Reidel, Dordrecht


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