1. Anile, A.M., Danese, L., De Zotti, G. and Motta, S. 1976. Ap.J. Lett., 205, L59.
2. Birkinshaw, M., Gull, S.F. and Northover, K.J.E. 1981. Submitted to MNRAS.
3. Boughn, S.P., Cheng, E.S. and Wilkinson, D.T. 1981. Ap.J.Lett., 243, L113.
4. Boynton, P.E. 1978. IAU Symposium 79. Ed. M.S. Longair and J. Einasto. p. 317. Reidel Publ.Co. Dordrecht, Holland.
5. Boynton, P.E. 1980. IAU Symposium 92. Ed. G.O. Abell and P.J. E. Peebles, p. 293. Reidel Publ.Co. Dordrecht, Holland.