1. Alpar, M. A., Cheng, A. F., Ruderman, M. A., Shaham, J. 1982,Nature,300, 728.
2. Anzer, U., Bonner, G., Mohaghan, J. J. 1986, MPI Green Report MPA 120, Munich.
3. Baym, G., Pethick, C. J., Sutherland, P. 1971,Astrophys. J.,170, 299.
4. Dewey, R. J., Maguire, G. A., Rawley, L. A., Stokes, G. R., Taylor, J. H. 1986,Nature,322, 712.
5. Ehlers, I., Walker, M. 1984, inProc. GRG 10 Conference, Eds B. Bertottiet al D. Reidel, Dordrecht, p. 125.