1. B. J. Gladman and P. Lee are in the Department of Astronomy, Cornell University, Ithaca, NY 14853, USA.
2. J. A. Burns is in the Departments of Astronomy and Theoretical and Applied Mechanics, Cornell University, Ithaca, NY 14853, USA.
3. M. Duncan is in the Department of Physics, Queen's University, Kingston, ON K7L 3N6, Canada.
4. H. F. Levison is with Geophysical, Astrophysical, and Planetary Sciences, Southwest Research Institute, 1050 Walnut Street, Suite 429, Boulder, CO 80302, USA.