1. Crovisier, J. & Schloerb, F. P. in Comets in the Post-Halley Era (eds Newburn, R. et al.) (Kluwer Academic, Dordrecht) (in the press).
2. Bockelée-Morvan, D. et al. Astr. Astrophys. 180, 253–262 (1987).
3. Colom, P., Despois, D., Bockelée-Morvan, D., Crovisier, J. & Paubert, G. in Observations of Recent Comets (ed. Huebner W. F. et al.) 80–85 (Southwest Research Institute, San Antonio, Texas, 1990).
4. Cosmovici, C. B. & Ortolani, S. Nature 310, 122–124 (1984).
5. Marconi, M. L. et al. Astr. J. 352, L17–L20 (1990).