The author wishes to thank the support staff and telescope operators of the Sierra Nevada and Loiano Observatories. Without their help the observations would not have been possible. He specially thanks Dr. M. Herranz de la Revilla and Dr. J. L. Moreno of the Institute of Astrophysics of Andalucia for their continuous assistance. The author is also grateful to Prof. V. Formisano for helpful discussion and to Mrs. S. Zampieri for her support during the manuscript preparation. Fundings were provided by ASI and CSIC grants.
© European Southern Observatory (ESO) 1998
Online publication: April 15, 1998