Comet Hale-Bopp shells expansion: A CCD study

Using a CCD camera attached to the 0.335 m and 0.20 m reflectors of S.A.S. Observatory (Novara, Italy), we followed the linear jets and shells of come t Hale-Bopp between May 1996 and May 1997. In addition to confirming the mo del of Sekanina and Bohenhardt (1997), the study of the linear jets provide d indications concerning the orientation of the comet's axis of rotation ov er time. The study of the shells revealed that the speed at which they move away from the nucleus was not constant. A periodic variation of the shell expansion velocity may not be excluded : if so, a possible precessional eff ect on the axis of rotation of the comet's nucleus could explain this behav ior.