1. Angel, J.R.P. 1977, Ap. J. 216, 1.
2. Bath, G.T., Pringle, J.E. and Whelan, J.A.J. 1980, M.N.R.A.S. 190, 185.
3. Becker, R.H. 1981, Ap. J. 251, 626.
4. Belserene, E.P. 1981, B.A.A.S., 13, 524.
5. Bonnet-Bidaud, J.M., Mouchet, M. and Motch, C. 1982, Astr. Astrophys., in press.