1. Jönsson, G., Kristiansson, K., Malmqvist, L.: Astrophys. Space Sci.7, 231 (1970)
2. Bogdanov, S.D., Ostroumov, V.I.: Isvestija Academii Nauk, U.S.S.R.38, 1811 (1974)
3. Beaujean, R., Sagebiel, H., Enge, W.: 15th Int. Cosmic Ray Conf., Plovdiv, Bulgaria,1, 329 (1977)
4. Garcia-Munoz, M., Mason, G.M., Simpson, J.A.: Astrophys. J. Letters201, L145 (1975)
5. Garcia-Munoz, M., Mason, G.M., Simpson, J.A.: Proceedings of the 15th Int. Cosmic Ray Conf., Plovdiv, Bulgaria,1, 301 (1977)