1. Abia, C., & Rebolo, R. 1989, preprint
2. Alcock, C., Fuller, G.M., & Mathews, G. J. 1987. Astrophys. J. 320, 439.
3. Applegate, J. H., Hogan, C.J., & Scherrer, R. J. 1987, Phys. Rev. D35, 1151.
4. Arimoto, N. 1989, in Evolutionary Phenomena in Galaxies, J. Beckman & B. E. J. Pagel (eds), Cambridge Univ. Press, p. 341.
5. Arimoto, N., & Yoshii, Y. 1987. Astron. Astrophys. 173, 23.