MULTICOLOR DEEP CCD PHOTOMETRIC-STUDY OF THE MODERATELY YOUNG SOUTHERN OPEN STAR-CLUSTERS NGC-3228, NGC-4103, NGC-5662 AND NGC-6087

CCD observations in the U B, V, R and I passbands have been used to ge nerate colour-colour and deep colour-magnitude diagrams for the young open star clusters NGC 3228, NGC 4103, NGC 5662 and NGC 6087. The samp le consists of about 2000 stars reaching down to V = 20 mag. The inter stellar extinction across the face of the imaged cluster regions seems to be uniform with average values of E(B - V) = 0.31, 0.34 and 0.22 m ag for NGC 4103; NGC 5662 and NGC 6087 respectively. The distances to the clusters NGC 3228, NGC 4103, NGC 5662 and NGC 6087 have been estim ated as 0.5 +/- 0.05, 2.0 +/- 0.3, 0.8 +/- 0.1 and 1.0 +/- 0.1 kpc res pectively, while the corresponding ages estimated from the theoretical stellar evolutionary isochrones fitted to the bright proper motion cl uster members are - 100, 30, 80 and 65 Myr respectively. Present data are consistent with the membership of the Cepheid variable V Cen to NG C 5662 and of S Norm to NGC 6087. Heavy contamination by foreground an d background field stars has unfortunately made it impossible to reach definite conclusions about the faint luminosity function, and the dat a do not reach faint enough limits to investigate pre-main sequence st ars.