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13. Conclusions

New UBVRI CCD photometry down to tex2html_wrap_inline3562 is presented for about 2000 stars altogether near the young open clusters NGC 3228, NGC 4103, NGC 5662 and NGC 6087. The present work in combination with earlier proper motion, radial velocity, photoelectric and spectroscopic observations leads to the following conclusions:

(i)
 Visual fitting of the ZAMS to the bluest envelope of the cluster sequence in the tex2html_wrap_inline3564; tex2html_wrap_inline3566; tex2html_wrap_inline3568 and tex2html_wrap_inline3570 diagrams over a broad range of V (tex2html_wrap_inline3574) gives distances of tex2html_wrap_inline3576 to NGC 4103, tex2html_wrap_inline3578 to NGC 5662 and tex2html_wrap_inline3580 to NGC 6087.
(ii)
 Reddening across the observed regions in NGC 4103, NGC 5662 and NGC 6087 seems to be uniform with mean values of E(B-V) = 0.31, 0.34 and 0.22 respectively. The law of interstellar extinction in the direction of these cluster regions is normal.
(iii)
 The present work supports earlier finding that the Cepheid variables V Cen and S Norm are probably members of NGC 5662 and NGC 6087 respectively.
(iv)
 Bertelli et al.  (1994) isochrones fitted to the bright cluster members in the tex2html_wrap_inline3584; tex2html_wrap_inline3586; tex2html_wrap_inline3588 and tex2html_wrap_inline3590 diagrams indicate that the oldest cluster in the sample is NGC 3228 with an age of tex2html_wrap_inline3592 and the youngest is NGC 4103 with an age of tex2html_wrap_inline3594 30 Myr. Both NGC 5662 and NGC 6087 are of nearly the same age, having values tex2html_wrap_inline3596 and 65 Myr respectively.
(v)
 It is difficult to separate cluster members from field stars on the basis of present observations alone; kinematic data are also required. Deep higher precision photometric and multi-object spectroscpic observations in the optical as well as in the near infrared are required to identify the PMS stars in the clusters under discussion.

We thank the Mount Stromlo and Siding Spring Observatories of the Australian National University for the allotment of observing time for this project and the referee Dr. J.J. Clariá for constructive comments. Financial support for RS provided by the then Department of Industry, Technology and Commerce of Australia, by the IAU and by the AAO are gratefully acknowledged. This research has made use of the Open Cluster Data Base compiled by J.-C. Mermilliod, Institut d'Astronomie de l'Université de Lausanne, Switzerland.


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