1. Avni, Y. & Bachall, J.L., 1980, Ap. J. ,235, 694.
2. Bartel, P.D., 1986, in Quasars ,eds. G. Swarup and B.K. Kapahi, Reidel, Dordrecht, 181.
3. Blandford, R.D., 1986, in Quasars ,eds. G. Swarup and B.K. Kapahi, Reidel, Dordrecht, 359.
4. Boyle, B.J., Fong, R., Shanks, T. & Peterson, B.A., 1987, M.N.R.A.S. 227, 717.
5. Cavaliere, A., Danese, L., De Zotti, G. & Franceschini, A., 1981, Astron. Astroph., 97, 269.