On heterotic model constraints
  The constraints imposed on heterotic compactifications by global consistency and phenomenology seem to be very finely balanced. We show that weakening these constraints, as was proposed in some recent works, is likely to lead to frivolous results. In particular, we construct an infinite set of such frivolous models having precisely the massless spectrum of the MSSM and other quasi-realistic features. Only one model in this infinite collection (the one constructed in arXiv:hep-th/0512149) is globally consistent and supersymmetric. The others might be interpreted as being anomalous, or as non-supersymmetric models, or as local models that cannot be embedded in a global one. We also show that the strongly coupled model of arXiv:hep-th/0512149 can be modified to a perturbative solution with stable SU(4) or SU(5) bundles in the hidden sector. We finally propose a detailed exploration of heterotic vacua involving bundles on Calabi-Yau threefolds with Z_6 Wilson lines; we obtain many more frivolous solutions, but none that are globally consistent and supersymmetric at the string scale. 
Vincent Bouchard and Ron Donagi