Parameters, limits and higher derivative type II string corrections

String theory in d dimensions has n+1=11-d parameters that may be thought of as being inherited from the geometry of an n+1 torus which may be used to construct the theory using dimensional reduction from eleven dimensions. We give the precise relationship between these parameters and the expectation values of the scalar fields that parameterise the E_{n+1} coset of the d dimensional theory. This allows us to examine all possible limits of the automorphic forms which occur as the coefficient functions of the higher derivative corrections to the d dimensional type II string effective action.