Table 1: Journal of observations of NGC 6996 and standard star fields together with calibration coefficients.
Field Filter Exposure time Seeing Air-mass
    [s] ['']  
NGC 6996 U $900\times2$ 180 10 2.2 1.091
1 frame B 600 60 10 2.1 1.062
Asiago V 300 30 5 2.2 1.052
8/11/02 I 300 30 5 2.3 1.045
NGC 6996 U 900 - - $\sim $3 1.368
2 frames B 600 - - $\sim $3 1.322
Teramo V 300 60 - $\sim $3 1.251
5-12-2002 I 180 60 - $\sim $3 1.219
NGC 6996 U 1200 120 - 1.9 1.007
1 frame B 800 80 8 1.8 1.018
Asiago V 600 60 6 1.8 1.027
18/09/03 I 400 40 4 1.7 1.037
PG 0231+051 U 800     2.5 1.348
Asiago B 300     2.4 1.324
8/11/02 V 60     2.2 1.316
  I 90     2.2 1.315
PG 2213-006 U 600     2.5 1.447
Asiago B 150     2.3 1.457
8/11/02 V 30     2.3 1.465
  I 30     2.3 1.472
Calibration $u_1 = +3.861 \pm 0.015$ $b_1 = +1.602 \pm 0.004$
coefficients $u_2 = -0.142 \pm 0.022$ $b_2 = +0.038 \pm 0.006$
Asiago u3 = +0.58 b3 = +0.29
8/11/02 $v_{1bv} = +1.003 \pm 0.014$ $i_1 = +1.691 \pm 0.044$
  $v_{2bv} = -0.016 \pm 0.018$ $i_2 = +0.057 \pm 0.043$
  v3 = +0.16 i3 = +0.08
  $v_{1vi} = +1.002 \pm 0.016$      
  $v_{2vi} = -0.013 \pm 0.016$      


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