  We reconsider the quantum vacuum interaction energy between two straight
parallel cosmic strings. This problem was discussed several times in an
approach treating both strings perturbatively and treating only one
perturbatively. Here we point out that a simplifying assumption made in [1] can
be justified and show that, despite the global character of the background, the
perturbative approach delivers a correct result. We consider the applicability
of the scattering methods, developed in the past decade for the Casimir effect,
for the cosmic string and find it not applicable. We calculate the scattering
T-operator on one string. Finally, we consider the vacuum interaction of two
strings when each carries a two dimensional delta function potential.
