1. Arkuszewski, W., Kopczyński, W. and Ponomariev, V. N.: 1975,Comm. Math. Phys. 45, 183.
2. Bardeen, J. M., Thorne, K. S. and Meltzer, D. W.: 1966,Astrophys. J. 145, 505.
3. Chapline, G. and Nauenberg, M.: 1976, ‘On the Possible Existence of Quark Stars’, preprint, UCSC 76/114.
4. Harrison, B. K., Thorne, K. S., Wakano, M. and Wheeler, J. A.: 1965,Gravitation Theory and Gravitational Collapse, University of Chicago Press, Chicago.
5. Hehl, F. W., Heyde, P., Kerlick, G. D. and Nester, J. M.: 1976,Rev. Mod. Phys. 48, 393.