  We present the results of a mixed action approach, employing dynamical
twisted mass fermions in the sea sector and overlap valence fermions, with the
aim of testing the continuum limit scaling behaviour of physical quantities,
taking the pion decay constant as an example. To render the computations
practical, we impose for this purpose a fixed finite volume with lattice size
$L\approx1.3$ fm. We also briefly review the techniques we have used to deal
with overlap fermions.
