We like to thank Drs. Uli Grossmann-Doerth, Horst Balthasar and Michael Knölker for their helpful comments on the manuscript and are very much indebted to Prof. Dr. W. Mattig for his support of this investigation. We also like to thank the referee for critical reading of the manuscript and for valuable suggestions that helped to improve the paper.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998