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1. Introduction

The colour magnitude diagrams (CMDs) of moderately young (tex2html_wrap_inline2248) open star clusters show the effect of stellar evolution at the brighter end and contain pre-main sequence (PMS) stars at the fainter end. Consequently, they provide a rare opportunity for reliable estimation of ages of high and low mass stars formed in a molecular cloud. So they can be used to determine the length of time over which star formation was active in a particular region and also to know which stars form first - high mass or low mass ones or if they formed together.

In order to address the questions raised above, we performed faint (tex2html_wrap_inline2254) multicolour CCD stellar photometry in five southern moderately young open star clusters, namely NGC 3228, NGC 4103, NGC 4755, NGC 5662 and NGC 6087. Based on these observations, the presence of PMS stars in NGC 4755 has been indicated for the first time by Sagar & Cannon (1995). They also found that massive (tex2html_wrap_inline2256) and low mass (tex2html_wrap_inline2258) stars in the cluster are formed nearly at the same time from a molecular cloud which might have existed for a period of about tex2html_wrap_inline2260. UBVRI photometry and similar analyses for the other four clusters are presented here.

Table 1 (click here) lists the relevant prior information (taken from Lyngå's (1987) catalogue) available on the clusters under discussion. Some of these parameters should probably be revised to take account of the new data presented here. The clusters all lie close to the galactic plane (tex2html_wrap_inline2264 and tex2html_wrap_inline2266) with moderate interstellar extinction (tex2html_wrap_inline2268) and at a galactocentric distance of tex2html_wrap_inline2270. Their ages and heliocentric distances range from tex2html_wrap_inline2272 to 130 Myr and 0.5 to 1.5 kpc respectively. The clusters under discussion are relatively compact objects with angular radii tex2html_wrap_inline2274. NGC 5662 and NGC 6087 each have one Cepheid variable. NGC 4103 contains an interesting eclipsing variable. All the clusters contain chemically peculiar stars which may be due to their youthfulness.

  table212
Table 1: Preliminary informations about the clusters under study. They are taken from the catalogue of Lyngå (1987)

Previous studies, the new observations and data reductions are described in the next three sections. The interstellar extinction, other photometric results and their implications for star formation processes are described in subsequent sections of this paper.


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