  We compute one-loop quantum corrections to gravitational couplings in the
effective action of four-dimensional heterotic strings where supersymmetry is
spontaneously broken by Scherk-Schwarz fluxes. We show that in both heterotic
and type II theories of this class, no moduli dependent corrections to the
Planck mass are generated. We explicitly compute the one-loop corrections to
the $\mathcal R^2$ coupling and find that, despite the absence of
supersymmetry, its contributions may still be organised into representations of
subgroups of the modular group, and admit a universal form, determined uniquely
by the multiplicities of the ground states of the theory. Moreover, similarly
to the case of gauge couplings, also the gravitational sector may become
strongly coupled in models which dynamically induce large volume for the extra
dimensions.
