This research was co-financed by Centro de Matemática da Universidade do Minho, Centro de Astrofísica da Universidade do Porto and grant BD/346/90-RM from JNICT, Portugal. The author wishes to thank Prof. J.P. De Greve, Prof. M.T.V.T. Lago and Prof. C. de Loore for valuable suggestions to improve the manuscript. The author also wishes to thank the referee for the critical yet constructive comments.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998