We acknowledge W. Heindl, I. Kreykenbohm, and R. Meier for useful discussions on the data analysis. We thank G. Morfill and R. Neuhäuser for granting the first author the time to finish this work. This work has been financed by DARA grant 50 OR 92054.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999