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.
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