We thank Profs. E. Herbst and M. Tanimoto for helpful comments on the manuscript and Dr. A. H. Saleck for valuable suggestions concerning the analysis. This work has been supported in part by the Deutsche Forschungsgemeinschaft (DFG) via Grant SFB 301 and the Ministry of Science of NRW. S.T. thanks fellowships from the Japan Society for the Promotion of Science (JSPS) and from DFG.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998