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Description
CCD observations in the B, V and I passbands have been used to generate colour-magnitude diagrams reaching down to V~19mag for two slightly characterized (Melotte 105 and Hogg 15) and two almost unstudied (Pismis 21 and Ruprecht 140) open clusters. The sample consists of about 1300 stars observed in fields of about 4'x4'. Our analysis shows that neither Pismis 21 nor Ruprecht 140 are genuine open clusters since no clear main sequences or other meaningful features can be seen in their colour-magnitude diagrams. Melotte 105 and Hogg 15 are open clusters affected by E(B-V)=0.42+/-0.03 and 0.95+/-0.05, respectively. Their distances to the Sun have been estimated as 2.2+/-0.3 and 2.6+/-0.08kpc, respectively, while the corresponding ages estimated from empirical isochrones fitted to the Main Sequence cluster members are ~350Myr and 300Myr, respectively. The present data are not consistent with the membership of the WN6 star HDE 311884 to Hogg 15.
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