1. Koeckelenbergh, A., Int. Astro. Union Circ. 1594 (1957).
2. Kraus, J. D., Int. Astro. Union Circ. 1596 (1957).
3. Seeger, Ch. L., Westerhout, G., and Conway, R. G., Int. Astro. Union Circ. 1599 (1957).
4. Hewish, A., Proc. Roy. Soc., A, 228, 238 (1955).