[marc0]

The "art of summing up divergent series" is a well established technique in quantum field theory (not only in string theory). Briefly, the underlying mathematical description of quantum field theory produces singularities. The latter are not physical (so their existence is due to our poor mathematical abilities) but they still contain useful physical information. This information can be extracted by "correctly" performing divergent sums. And its results are verified by experiment, e.g. in higher order corrections to particle masses.