  We compute the supersymmetry constraints on the R^4 type corrections in
maximal supergravity in dimension 8, 6, 4 and 3, and determine the tensorial
differential equations satisfied by the function of the scalar fields
multiplying the R^4 term in the corresponding invariants. The second order
derivative of this function restricted to the Joseph ideal vanishes in
dimension lower than six. These results are extended to the d^4 R^4 and the d^6
R^4 corrections, based on the harmonic superspace construction of these
invariants in the linearised approximation. We discuss the solutions of these
differential equations and analysis the consequences on the non-perturbative
type II low energy string theory effective action.
