Heterotic fluxes and supersymmetry
  We show that the formal alpha' expansion for heterotic flux vacua is only sensible when flux quantization and the appearance of string scale cycles in the geometry are carefully taken into account. We summarize a number of properties of solutions with N=1 and N=2 space-time supersymmetry. 
Ilarion V. Melnikov, Ruben Minasian, and Savdeep Sethi
hep-th