  On-shell superspace techniques are used to quantify R-symmetry violation in
type IIB superstring theory amplitudes in a flat background in ten dimensions.
This shows the existence of a particularly simple class of non-vanishing
amplitudes in this theory which violate R-symmetry maximally. General
properties of the class and some of its extensions are established which at
string tree level are shown to determine the first three non-trivial effective
field theory contributions to all multiplicity. This leads to a natural
conjecture for the exact analytic part of the first two of these.
