1. Branson, N.J.B.A., Elsmore, B., Pooley, G.G., and Ryle, M., 1972, M.N.R.A.S., 156, 377.
2. Christiansen, N.N., 1953, Nature, 171, 831.
3. Cohen, M.H., Gundermann, E.J., and Harris, D.E., 1967, Astrophys. J. 150, 767.
4. Colla, G., Fanti, C., Fanti, R., Gioia, I., Lari, C., Lequeux, J., Lucas, R., Ulrich, M.-H., 1975, Astron, and Astrophys., 38, 209.
5. Cooper, B.F.C., Price, R.M., and Cole, D.J., 1965, Aust. J. Phys., 18, 589.