1. Alvarez, W. and Muller, R. A.: 1984, Nature 308, 718?20.
2. Anderson, J. D. and Standish Jr., E. M.: 1986, Dynamical Evidence for Planet X, in: R. Smoluchowski et al. (eds.), The Galaxy and the Solar System, University of Arizona Press, Tucson.
3. Harrington, R. S.: 1986, quoted by Anderson and Standish (1986).
4. Harrington, R. S.: 1988, Astron. J. 94(4), 1476?1478.
5. Harrington, R. S. and van Flandern, T. C.: 1979, Icarus 39, 131?136.