1. Bash, F.: 1986, ‘Present, past and future velocity of nearby stars: the path of the Sun in 108 years’, in The Galaxy and the Solar System, R. Smoluchowski, J.N. Bahcall and M.S. Matthews (Eds.), p. 35. The Univ. Arizona Press: Tucson.
2. Davis M., Hut, P. and Muller, R.A.: 1984, ‘Extinction of species by periodic comet showers’, Nature, 308, 715.
3. Delsemme, A.H.: 1986, ‘Cometary evidence for a solar companion’, in The Galaxy and the Solar System, R. Srmoluchowski, J.N. Bahcall and M.S. Matthews (Eds.), p. 173. The Univ. Arizona Press: Tucson.
4. Donn, B.: 1976, ‘Ctilrmets, interstellar clouds and star clusters’, in The Study of Comets, B. Donn, M. Mumma, W. Jackson, M. A Hearn and R. Harrington (Eds.), NASA SP-373, p. 611.
5. Duncan, M., Quinn, T. and Tremaine, S.: 1987, ‘The formation and extent of the Solar System comet cloud’, Astron. J., 94,1330.