5 Conclusions

We have presented CCD BVI Johnson-Cousins photometry of stars in the fields of the open clusters Melotte 105, Hogg 15, Pismis 21 and Ruprecht 140, all of them located towards the central parts of the Galaxy. The photometric data on Melotte 105, Hogg 15 and Pismis 21 supersede previous measurements in these clusters, thus allowing us to extend the corresponding MSs down to V $\sim$ 19.

Melotte 105 and Hogg 15 were found to be genuine open clusters, while Pismis 21 and Ruprecht 140 appeared not to be open clusters but simple density fluctuations above the background level. The average ratio E(V-I)/E(B-V) derived for the two genuine clusters was found to be 1.28 $\pm$ 0.05, in very good agreement with the value 1.25 generally adopted for the normal extinction law. The present photometric data indicate that Melotte 105 and Hogg 15 are moderately young (300-350 Myr) open clusters, located approximately in the direction towards the Galactic centre at 2.2 kpc and 2.6 kpc from the Sun, respectively. The difference between the photometric reddening in Melotte 105 and the one previously determined from integrated spectroscopy by Santos & Bica (1993) and Ahumada et al. (2000), is most probably due to the fact that the bright blue straggler (V = 11.29) contributes to the integrated light with a larger flux than those of the remaining cluster members. On the other hand, the present CCD data are not consistent with the membership of the WN6 star HDE 311884 to Hogg 15, which explains why this cluster is now found to be much older and closer to the Sun than previously thought.

Acknowledgements

We wish to thank the Director, staff and technicians of Las Campanas Observatory for the allocation of observing time and for their kind assistance and hospitality. Thanks are also due to an anonymous referee for making useful suggestions which improved this paper. We gratefully acknowledge financial support from the Argentinian institutions CONICET, Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT) and SECYT (National University of Córdoba).