I am deeply indebted to Drs. R.E. Nather, D. O'Donoghue, E.L. Robinson and B. Warner who gave me access to their data archives. Without their support this study could have not been performed. I am also grateful to an anonymous referee for an extraordinarily helpful and considerate report. This work was supported by grants of the Conselho Nacional de Desenvolvimento Científico e Tecnológico (No. 301784/95-5) and the Deutsche Forschungsgemeinschaft (No. Br. 706/8-1/8-2).
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000