The authors are very grateful to M. Stix for valuable discussions, and to C. Fang for his contribution in making the computation program. Great thanks are also due to the referee, R.W. Noyes, for his helpful comments and suggestions. M.D.D. would like to thank the staff of the Kiepenheuer-Institut for their kind hospitality supporting his visit there.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998