This work has been supported by the Brazilian institutions: FAPEMIG, CNPq, FINEP, CAPES. LPRV acknowledges (CNPq) support received while being a visiting scientist at the Department of Astronomy, University of Wisconsin, Madison, and is grateful for the hospitality received there from Aug 1993 to Oct 1994, while part of this paper was completed. Fruitful discussions with Dr. Jens Viggo Clausen and a careful reading by Dr. Dave Stickland are gratefully acknowledged. This research has made use of the Simbad data base, operated by the CDS, Strasbourg, France. A substantial part of the computations was done under LINUX (a GNU Public Licence UNIX operating system clone).
© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998