  In this paper, we study instanton corrections in the N=2* gauge theory by
using its description in string theory as a freely-acting orbifold. The latter
is used to compute, using the worldsheet, the deformation of the Yang-Mills
action. In addition, we calculate the deformed instanton partition function,
thus extending the results to the non-perturbative sector of the gauge theory.
As we point out, the structure of the deformation is extremely similar to the
Omega-deformation, therefore confirming the universality of the construction.
Finally, we comment on the realisation of the mass deformation using physical
vertex operators by exploiting the equivalence between Scherk-Schwarz
deformations and freely-acting orbifolds.
