I wish to thank P. Hakala, J. Huovelin, K. Muinonen, O. Vilhu and the referee, K. Wood, for their constructive criticism and useful comments. This research was supported by the Space Research Programme at University of Helsinki Observatory, supported by Academy of Finland. This research has made use of NASA's Astrophysics Data System Bibliographic Services.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001