1. Trimble, V. L., and Thorne, K. S., Astrophys. J., 156, 1013 (1969).
2. Batten, A. H., Pub. Dom. Astrophys. Obs. Victoria, 13, 119 (1967).
3. Maestre, L. A., and Wright, J. A., Astrophys. J., 131, 119 (1960).
4. Muller, P., J. des Observateurs, 40, 48 (1957).
5. Zel'dovich, Ya. B., and Guseynov, O. H., Astrophys. J., 144, 841 (1965).