We are grateful to Drs. B. Cotton and S. Spangler for very helpful comments on the manuscript of this paper and to Dr. H. Andernach for the suggestions on the compilation of the flux densities. We made use of the database CATS (Verkhodanov et al. 1997) of the Special Astrophysical Observatory. We acknowledge the Italian Ministry for University and Scientific Research (MURST) for partial financial support (grant Cofin98-02-32). Part of this work was supported by European Commission, TMR Programme, Research Network Contract ERBFMRXCT97-0034 "CERES".
© European Southern Observatory (ESO) 1999
Online publication: April 28, 1999