  Assuming that the Standard Model arises from the $E_8\times E_8$ Heterotic
Superstring, we try to solve the discrepancy between the unification scale
predicted by this theory ($\approx g_{GUT}\times 5.27\cdot 10^{17}$ GeV) and
the value deduced from LEP experiments ($\approx 2\cdot 10^{16}$ GeV). This
will allow us to predict the presence at low energies of three generations of
supersymmetric Higgses and vector-like colour triplets.
