  It is well known that under T-duality the sigma model dilaton (which is
normally thought to be related to the string coupling constant through the
simple formula $\kappa = exp <\phi >$), undergoes an additive shift. On the
other hand, Kugo and Zwiebach, using a simplified form of string field theory,
claim that the string coupling constant does not change under the T-duality.
Obviously, what seems to happen is that two different coupling constants,
associated to different dilatons, are used. In this contribution we shall try
to clarify this, and related issues.
