1. Bahcall, J. N. and Casertano, S.: 1985,Astrophys. J. 293, L10.
2. Bosma, A. P.: 1978, Thesis, University of Groningen, The Netherlands.
3. Erickson, L. K., Gottesman, S. T., and Hunter, J. H.: 1987,Nature 325, 779.
4. Ortega, V. G. and Pacheco, J. A. de Freitas: 1986,Rev. Mexicana Astron. Astrof. 12, 97.
5. Ortega, V. G. and Pacheco, J. A. de Freitas: 1988, (in prep.)