ML acknowledges the kind hospitality of the Astronomical Observatories of Brera (Milan) and Padua during her PhD thesis and the support by the European Community under TMR grant ERBFMBI-CT97-2804. RR acknowledges the hospitality of the Asiago and Padua Astronomical Observatories where part of this study has been carried out. CC whishes to acknowledge the friendly hospitality and stimulating environment provided by MPA in Garching where this paper has been completed during leave of absence from the Astronomy department of the Padua University. ML, AB and CC acknowledge the support by the European Community under TMR grant ERBFMRX-CT96-0086.
© European Southern Observatory (ESO) 1999
Online publication: April 19, 1999