I thank Dr. J. L.Moreno and Dr. J. Rodrigueez of Institute of Astrophysics of Andalucia for their assistance. The author wishes to thank Dr. M. Wolff for his detailed review that resulted in a substantial improvement respect to the earlier version of manuscript. The help of Mrs. S. Zampieri in the manuscript preparation is also acknowledged. Funding were provided by ASI and CSIC grants.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000