Background constraints in the infinite tension limit of the heterotic   string
  In this work we investigate the classical constraints imposed on the supergravity and super Yang-Mills backgrounds in the $\alpha'\to 0$ limit of the heterotic string using the pure spinor formalism. Guided by the recently observed sectorization of the model, we show that all the ten-dimensional constraints are elegantly obtained from the single condition of nilpotency of the BRST charge. 
Thales Azevedo and Renann Lipinski Jusinskas
hep-th