We are grateful to the referee (R. Tylenda) for his critical comments, which improved the paper considerably. We thank the night assistants and the whole technical staff of the Astronomical Observatory of Asiago at Cima Ekar (Italy) for their competence and patience. This paper is partially based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Centro Galileo Galilei of the CNAA (Consorzio Nazionale per l'Astronomia e l'Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000