We have presented new CCD VI photometry for Westerlund1 and integrated spectra in the visible and near-IR ranges for Westerlund1 and 2. The analysis of the observed data allowed us to improve the knowledge of the fundamental properties for both clusters. Westerlund1 is a compact open cluster with an age of Myr located at kpc from the sun. It is affected by a strong interstellar visual absorption (Av=12.9 mag), which, as far as we aware, converts it into the most reddened open cluster in the Galaxy studied in detail. From the CaII triplet lines we conclude that the cluster has a nearly solar metallicity. Westerlund 1 is a massive cluster, but it contains several luminous stars confined in a relatively small volume.
For Westerlund2 we estimated an age of 2-3 Myr, a foreground reddening together with evidence of some amount of internal reddening. We derived a revised distance for Westerlund2 of kpc, with the help of complementary observations taken from the literature. Westerlund2 is quite massive, but not as much as NGC 3603.
The formation of massive globular-like clusters apparently requires much larger star-forming complexes than are now usually found in the Milky Way; they form in the densest parts of such complexes, and the molecular clumps in which they are embedded are compressed in part by the effects of earlier star formation (Larson 1993). NGC 3603 possibly satisfies those conditions. The concentration of supergiants in the central region of Westerlund1 is due to a somewhat smaller scale enhanced star formation event.
We acknowledge use of the CCD and data acquisition system at CASLEO supported under U.S. National Science Foundation grant AST-90-15827 to R.M. Rich. We are grateful to the staff at CASLEO and Las Campanas, for their kind hospitality and assistance during the observing runs. We are also very indebted to the referee Dr. J.C. Mermilliod whose valuable comments and suggestions helped us to improve the manuscript. This work was partially supported by the institutions CNPq and FINEP (Brazil), and CONICET and CONICOR (Argentina). We also acknowledge support from the Vitae and Antorchas foundations.