  We study the phase structure of mixed-action QCD with two Wilson sea quarks
and two chiral valence quarks, starting from the chiral lagrangian. A priori,
the effective theory allows for a rich phase structure, including a phase with
a condensate made of sea and valence quarks. Because this would lead to mass
eigenstates that are admixtures of sea and valence fields, pure-sea correlation
functions would depend on valence quark masses, in contradiction with the
actual setup of mixed-action simulations. Using properties of the chiral Dirac
operator, we prove that such a phase does not occur, and that this leads to
bounds on low-energy constants.
