1. Amitai-Milchgrub, A., Piran, T., and Shabam, J.: 1979,Nature 279, 505.
2. Ciatti, F., Mammano, A., Iijima, T., and Vittone, A.: 1983,Astron. Astrophys., in press.
3. Cogotti, R., Ruffini, R., and Song, D. J.: 1983,Astron. Astrophys., in press.
4. Crampton, D. and Hutchings, J. B.: 1981,Astrophys. J. 251, 604.
5. Crampton, D., Cowley, A. R., and Hutchings, J. B.: 1980,Astrophys. J. 235, 431.