This work was supported by Swedish Natural Science Research Foundation (NFR) and partly by the Sonderforschungsbereich (SFB) 328 of the University of Heidelberg. I thank Max Camenzind and Stefan Appl for discussions. Lennart Lindegren is acknowledged for putting the mathematical software packet Mathcad at my disposal. The paper also benefited from the conversation with the referee Jonathan Ferreira.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998