  We compute at one loop the two-point functions of massless scalars in the
Neumann-Dirichlet open-string sector of the type IIB orientifold compactified
on $T^2\times T^4/\mathbb{Z}_2$, when $\mathcal{N}=2$ supersymmetry is
spontaneously broken. This is done by evaluating correlation functions of
``boundary-changing vertex operators'' which are analogous to correlators of
twist fields for closed strings. We use our results to compute the mass
developed at one loop by the moduli fields arising in the Neumann-Dirichlet
sector.
