Reduced modular symmetries of threshold corrections and gauge coupling   unification
  We revisit the question of gauge coupling unification at the string scale in orbifold compactifications of the heterotic string for the supersymmetric Standard Model. In the presence of discrete Wilson lines threshold corrections with modular symmetry that is a subgroup of the full modular group arise. We find that reduced modular symmetries not previously reported are possible. We conjecture that the effects of such threshold corrections can be simulated using sums of terms built from Dedekind eta functions to obtain the appropriate modular symmetry. For the cases of the Z_8-I orbifold and the Z_3 x Z_6 orbifold it is easily possible to obtain gauge coupling unification at the "observed" scale with Kahler moduli T of approximately one. 
David Bailin and Alex Love
hep-th