  A naive dimensional reduction of the $N=1, D=10$ supergravity theory that
naturally arises in five-brane models is used to determine the r\^ole of two
fields which are basic ingredients of string models: the dilaton and, among the
moduli, the breathing mode. It is shown that, under the duality transformation
that relates five-branes and strings, these two fields exchange the r\^oles of
10-dimensional dilaton and radius of the compact manifold. A description of
this phenomenon in terms of the linear multiplets of the 4-dimensional
supergravity is also presented.
