  We investigate the dependence of the gauge couplings on the dilaton field in
string effective theories at the one--loop level. First we resolve the
discrepancies between statements based on symmetry considerations and explicit
calculations in string effective theories on this subject. A calculation of the
relevant one--loop scattering amplitudes in string theory gives us further
information and allows us to derive the exact form of the corresponding
effective Lagrangian. In particular there is no dilaton dependent one--loop
correction to the holomorphic $f$--function arising from massive string modes
in the loop. In addition we address the coupling of the antisymmetric tensor
field to the gauge bosons at one--loop. While the string S--matrix elements are
not reproduced using the usual supersymmetric Lagrangian with the chiral
superfield representation for the dilaton field, the analogue Lagrangian with
the dilaton in a linear multiplet naturally gives the correct answer.
