This work was accomplished during the stage of S.L. at Astronomical Observatory of Capodimonte (OAC) in Naples. S.L. wishes to thank the OAC staff for support as well as the Russian Fund of Fundamental Research for the grant 99-02-17184. The authors thank Dr. K. Readon for critical reading the manuscript. The authors also wish to thank the anonymous referee for his helpful remarks.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000