Heterotic supergeometry revisited
  The superspace geometry relevant to the heterotic string is reviewed from the point of view of the off-shell supermultiplet structure of $N=1,d=10$ supergravity. The anomaly-modified seven-form Bianchi identity is analysed at order $\a'^3$ and shown to admit a complete solution. The corresponding $\a'^3$ deformation of the dimension-zero torsion tensor is derived and shown to obey the appropriate cohomological constraint. 
P.S. Howe