1. Amnuel, P.R.: 1993,Monthly Not. Roy. Astr. Soc. 261, 263.
2. Amnuel, P.R.: 1994,Astrophys. Space Sci. 219, 117.
3. Amnuel, P.R., Guseinov, O.H. and Rustamov, Yu.S.: 1989,Astrophys. Space Sci. 154, 21.
4. Bond, G.: 1994, in:Proc. of Asymmetrical PN Conference, Oranim, Israel.
5. Björkman, J.E. and Cassinelli, J.P.: 1992, in: L. Drissen, C. Leitherer and A. Nota (eds.),Nonisotropic and Variable Outflows from Stars, ASP, San Francisco, p. 88.