  Using the current precision electroweak data, we look for the minimal
particle content which is necessary to add to the standard model in order to
have a complete unification of gauge couplings and gravity at the weakly
coupled heterotic string scale. We find that the addition of a vector-like
fermion at an intermediate scale and a non-standard hypercharge normalization
are in general sufficient to achieve this goal at two-loop level. Requiring the
extra matter scale to be below the TeV scale, it is found that the addition of
three vector-like fermion doublets with a mass around 700 GeV yields a perfect
string-scale unification, provided that the affine levels are $(k_Y, k_2
,k_3)=(13/3, 1, 2) $, as in the $SU(5) \times SU(5)$ string-GUT. Furthermore,
if supersymmetry is broken at the unification scale, the Higgs mass is
predicted in the range 125 GeV - 170 GeV, depending on the precise values of
the top quark mass and $\tan \beta$ parameter.
