We thank G. Stellmacher and R. Muller for their help and their comments on this work. The observations have been obtained at the VTT/SPO thanks to the excellent work provided by the staff there. Part of this work was made in the frame of the NATO CRG 940291, with the support of INSU and CNRS (France). FB and RMH have been supported by the Spanish DGICYT (project no 91-0530).
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997