  We determine the units and numerical values for a class of couplings in the
effective theory of perturbative heterotic string vacua, with the emphasis on
the correct translation between the canonical gauge coupling g and Planck scale
M_Planck ~ 1.2 x 10^19 GeV as used in the effective theory description and the
string coupling g_string and string tension alpha' as used in the S-matrix
amplitude calculation. In particular, we determine the effective couplings in
the superpotential and revisit the Fayet-Iliopoulos (FI) term in a class of
models with an anomalous U(1). We derive the values of the effective Yukawa
couplings (at the third and fourth order) after the restabilization of vacuum
along a particular F- and D-flat direction and show that they are comparable in
magnitude. The result corrects results quoted in the literature, and may have
implications for the string derived phenomenology, e.g., that of fermion
textures.
