  We present a unified point of view on the different methods available in the
literature to extract gauge theory renormalization constants from the
low-energy limit of string theory. The Bern-Kosower method, based on an
off-shell continuation of string theory amplitudes, and the construction of
low-energy string theory effective actions for gauge particles, can both be
understood in terms of strings interacting with background gauge fields, and
thus reproduce, in the low-energy limit, the field theory results of the
background field method. We present in particular a consistent off-shell
continuation of the one-loop gluon amplitudes in the open bosonic string that
reproduces exactly the results of the background field method in the Feynman
gauge.
