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2. For references see [1] and Wilson, B. G.: 1968, ISAS Report, Institute of Space and Aeronautical Science, University of Tokyo, No. 428.
3. Bowyer, C. S., Field, G. B., and Mack, J. E.: 1968, Nature 217, 32.
4. Henry, R. C., Fritz, G., Meekins, J. F., Friedman, H., and Byram, E. T.: 1968, Astrophys. J. 153, L199.
5. Grader, R. J., Hill, R. W., and Seward, F. D.: 1969, Sky and Telescope, Feb. issue, p. 79.