  Effects of ion mobility and positron fraction on solitary waves of envelop of
laser field and potential of electrostatic field in weak relativistic
electron-positron-ion plasma are investigated. The parameter region for the
existence of solitary waves is obtained analytically, and the reasonable choice
of parameters is clarified. Both cases of mobile and immobile ions are
considered. It is found that the amplitudes of solitary waves in the former
case are larger compared to the latter case. For small plasma density, the
localized solitary wave solutions in terms of approximate perturbation
analytical method are consistent well with that by exact numerical
calculations. However as the plasma density increases the analytical method
loses its validity more and more. The influence of the positron fraction on the
amplitudes of solitary waves shows a monotonous increasing relation.
Implication of our results to the particle acceleration is also discussed
briefly.
