The authors would like to acknowledge Prof. C.D. Lin for giving them his computer code, Drs. J.C. Hénoux, S. Sahal and A. Spielfiedel for fruitful discussions and the referee for interesting suggestions. The computations were performed on the Convex and Dec Alpha stations of the Computational Centre of Paris Observatory.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998