1. Bahcall, J.N. and Soneira, R.M. 1980 preprint.
2. Bonoli, F., Braccesi, A., Marano, B., Merighi, R., and Zitelli, V. 1980 Submitted to Astr. and Ap.
3. Cavaliere, A., Danse, L., DeZotti, G., and Franceschini, A. 1979, Astr. and Ap., 79, 169.
4. Fabian, A.C. 1975, Mon.Nat.R.Astr.Soc., 172, 149.
5. Felton, J.E. and Morrison, P. 1966, Ap.J., 146, 686.