  We study the non-perturbative corrections generated by exotic instantons in
U(N) gauge theories in eight and four dimensions.
  As it was shown previously, the eight-dimensional prepotential can be
resummed using a plethystic formula showing only a dependence from the center
of mass and from a
  U(1) gauge factor. On the contrary, chiral correlators in eight and four
dimensions display a non-trivial dependence from the full gauge group.
Furthermore the resolvent, the generating function for the eight and four
dimensional correlators, can be written in a compact form both in the eight and
four dimensional cases.
