  We reconsider the general properties of gravitational lensing effects induced
by cosmic string systems, taking into account their equation of state and
motion equations. We explicitly show that the deflection patterns induced by a
string is equivalent to the one of a lineic and static distribution of matter.
We then rigorously show that the convergence part of the deformation field is
always zero, except on the intersection of the string worldsheet and the
observer past light cone, extending previous results obtained in peculiar
cases. Phenomenological consequences of this result on multiple image systems
are investigated.
