The authors thank the anonymous referee for his/her useful comments. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The Italian MURST is thanked for financial support.
© European Southern Observatory (ESO) 2000
Online publication: June 26, 2000