The numerical computations have been performed on the Cray C98 (IDRIS, CNRS, Orsay). We acknowledge financial support from the GDR 131 from CNRS. S. Tomczyk acknowledges support from the US National Science Foundation through base funding of HAO/NCAR. We wish to thank the anonymous referee for his constructive remarks.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998