  E3-instantons that generate non-perturbative superpotentials in IIB N=1
compactifications are more frequent than currently believed. Worldvolume fluxes
will typically lift the E3-brane geometric moduli and their fermionic
superpartners, leaving only the two required universal fermionic zero-modes. We
consistently incorporate SL(2, Z) monodromies and world-volume fluxes in the
effective theory of the E3-brane fermions and study the resulting zero-mode
spectrum, highlighting the relation between F-theory and perturbative IIB
results. This leads us to a IIB derivation of the index for generation of
superpotential terms, which reproduces and generalizes available results.
Furthermore, we show how worldvolume fluxes can be explicitly constructed in a
one-modulus compactification, such that an E3-instanton has exactly two
fermonic zero-modes. This construction is readily applicable to numerous
scenarios.
