This investigation is supported by the University of Bologna (Funds for selected research topics). We acknowledge the use of the BFOSC instrument of the Bologna Astronomical Observatory. We thank M. Tavani, G.M. Beskin and M. Corwin for useful comments and suggestions. We also thank A. Bragaglia, P. Focardi, A. Comastri and G. Tozzi for having given us part of their observational time.
© European Southern Observatory (ESO) 1997
Online publication: March 26, 1998