The authors would like to thank Gabriele Born and Walter Teuschel for help with the experiments and valuable discussions. This work has been supported by the Danish Natural Science Research Council and by the Deutsche Forschungsgemeinschaft (DFG grant Mu 1164/3-1). H. M. is supported by a grant from the Max Planck Society to Th. H.
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999