1. Baart, E. E., Barrow, C. H., and Lee, R. T.: 1966,Nature 211, 808.
2. Barrow, C. H. and Baart, E. E.: 1967,Nature 213, 165.
3. Bigg, E. K.: 1964,Nature 203, 1008.
4. Brown, G. W., Carr, T. D., and Block, W. F.: 1968,Astrophys. Letters 1, 89.
5. Burke, B. F., Firor, J. W., Franklin, K. L., and Wells, H. W.: 1955,Carnegie Institution of Washington Year Book No. 54.