Ten-dimensional super-Yang-Mills with nine off-shell supersymmetries

After adding 7 auxiliary scalars to the d=10 super-Yang-Mills action, 9 of the 16 supersymmetries close off-shell. In this Letter, these 9 supersymmetry generators are related by dimensional reduction to scalar and vector topological symmetry in N=2, d=8 twisted super-Yang-Mills. Furthermore, a gauge-invariant superspace action is constructed for d=10 super-Yang-Mills where the superfields depend on 9 anticommuting θ variables.