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Astron. Astrophys. Suppl. Ser. 140, 135-143

Strömgren photometry of the open clusters NGC 6134 and
NGC 3680[*]

H. Bruntt1 - S. Frandsen1 - H. Kjeldsen1,2 - M.I. Andersen3

Send offprint request: H. Bruntt

1 - Institute for Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark
2 - Theoretical Astrophysics Center, Danmarks Grundforskningsfond, Aarhus University, DK-8000 Aarhus C, Denmark
3 - Nordic Optical Telescope Apartado 474, Santa Cruz de La Palma, E-38700, Spain

Received May 17; accepted June 9, 1999


We present Strömgren photometry of the two southern open clusters NGC 6134 and NGC 3680. The field of NGC 6134 was observed with different shifts of the telescope and rotations of the CCD camera in order to detect and remove any large scale flat field errors using the technique described by Andersen et al. (1995). By using calibrations of the Strömgren indices we find the metallicity and interstellar reddening of the clusters: For NGC 6134 the results are [Fe/H] $=
0.28\pm0.02$ and $E(b-y) = 0.263\pm0.004$ and for NGC 3680 we find ${\rm [Fe/H]}=0.09\pm0.02$ and $E(b-y) = 0.048\pm0.011$. When comparing the observed colour-magnitude diagrams theoretical isochrones we find that models which include overshooting in the convective cores agree with the extended hook after the turnoff. From the isochrone fit we find ages of $0.69\pm0.10$ Gyr and $1.45\pm0.15$ Gyr for NGC 6134 and NGC 3680 respectively. Strömgren photometry of six known $\delta $ Scuti stars in NGC 6134 is presented.

Key words: techniques: photometric -- open clusters and associations: individual: NGC 6134, NGC 3680 -- $\delta $ Sct

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