  The effective action of N=2 supersymmetric 5-dimensional supergravity arising
from compactifications of M-theory on Calabi-Yau threefolds receives
non-perturbative corrections from wrapped Euclidean membranes and fivebranes.
These contributions can be interpreted as instanton corrections in the 5
dimensional field theory. Focusing on the universal hypermultiplet, a solution
of this type is presented and the instanton action is calculated, generalizing
previous results involving membrane instantons. The instanton action is not a
sum of membrane and fivebrane contributions: it has the form reminiscent of
non-threshold bound states.
