Heterotic supersymmetry, anomaly cancellation and equations of motion

We show that the heterotic supersymmetry (Killing spinor equations) and the anomaly cancellation imply the heterotic equations of motion in dimensions five, six, seven, eight if and only if the connection on the tangent bundle is an instanton. For heterotic compactifications in dimension six this reduces the choice of that connection to the unique SU(3) instanton on a manifold with stable tangent bundle of degree zero.Comment: LaTeX 2e, 11 pages, exposition clarified, reference added, final version to appear in Phys. Lett.