1. Bowden, C.C.G. et al. (1993), Proc. 23rd. ICRC, Calgary, 1, 294.
2. Cawley, M.F., Weekes, T.C. (1984) Astron. Astrophys. 133,80.
3. Cawley, M.F. et al. (1985), Proc. 19th ICRC, La Jolla, 1, 264.
4. Chudakov, A.E. et al. (1963) Proc. 8th ICRC, Jaipur, 4, 199.
5. De Jager, O.C., et al. (1994), Nature, (in press).