  We study electron-positron pair creation by a strong and constant electric
field superimposed with a weaker transversal plane wave which is incident
perpendicularly (or under some angle). Comparing the fully non-perturbative
approach based on the world-line instanton method with a perturbative expansion
into powers of the strength of the weaker plane wave, we find good agreement -
provided that the latter is carried out to sufficiently high orders. As usual
for the dynamically assisted Sauter-Schwinger effect, the additional plane wave
induces an exponential enhancement of the pair-creation probability if the
combined Keldysh parameter exceeds a certain threshold.
