1. Axelrod, T. S.: 1988, Lecture Notes in Physics
305, 3754
2. Baade, W. and Zwicky, F.: 1934, Proc. Nat. Acad. Sci.
20, 254
3. Bailyn, C. D. and Grindlay, J. E.: 1990, Astrophys. J.
353, 159
4. Bandiera, R., Pacini, F., and Salvati, M.: 1984, Astrophys. J.
285, 134
5. Bertelli, G., Bressan, A. G., and Chiosi, C.: 1985, Astron. Astrophys.
150, 33