We acknowledge financial support from the CONICET (Argentina) and CNRS (France) through their Argentina-France cooperative science program. We thank the referee Bert Van den Oord for his detailed comments which help to improve the readability of this paper. We thank Mona Hagyard, Jean Rayrole and Brigitte Schmieder for the magnetic field observations. P. D. thanks the staff at IAFE for his hospitality and C.H.M. thanks the staff at DASOP in turn.
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998