  Based on the structure of the on-shell linearized superspace of type IIB
supergravity, we argue that there is a non--BPS 16 derivative interaction in
the effective action of type IIB string theory of the form (t_8 t_8 R^4)^2,
which we call the R^8 interaction. It lies in the same supermultiplet as the
G^8 R^4 interaction. Using the KLT relation, we analyse the structure of the
tree level eight graviton scattering amplitude in the type IIB theory, which
leads to the R^8 interaction at the linearized level. This involves an analysis
of color ordered multi-gluon disc amplitudes in the type I theory, which shows
an intricate pole structure and transcendentality consistent with various other
interactions. Considerations of S-duality show that the R^8 interaction
receives non-analytic contributions in the string coupling at one and two
loops. Apart from receiving perturbative contributions, we show that the R^8
interaction receives a non-vanishing contribution in the one
D-instanton-anti-instanton background at leading order in the weak coupling
expansion.
