  We study a lattice QCD mixed action with overlap valence quarks on two
flavours of Wilson maximally twisted mass sea quarks. Employing three different
matching conditions to relate both actions to each other, we investigate the
continuum limit by using three values of the lattice spacing ranging from 0.05
fm to 0.08 fm. A particular emphasis is put on the effect on physical
observables of the topological zero modes appearing in the valence overlap
operator. We estimate the region of parameter space where the contribution from
these zero modes is sufficiently small such that their effects can be safely
controlled and a restoration of unitarity of the mixed action in the continuum
limit is reached.
