1. Margon, B. et al. Astrophys. J. Lett. (in the press).
2. Huang, S-S. Astrophys. J. 171, 549 (1972).
3. Luminet, J-P. preprint, (1978).
4. Payne, D. & Eardley, D. Astrophys. Lett. 19, 39 (1977).
5. Ciatti, F., Mammano, A. & Vittone, A. IAU Circ. No. 3305 (1978).