Summary: The coupling of non-Hermitian \({\mathcal P}{\mathcal T} \)-symmetric Hamiltonians to standard Hermitian Hamiltonians, each of which individually has a real energy spectrum, is explored by means of a number of soluble models. It is found that in all cases the energy remains real for small values of the coupling constant, but becomes complex if the coupling becomes stronger than some critical value. For a quadratic non-Hermitian \({\mathcal P}{\mathcal T} \)-symmetric Hamiltonian coupled to an arbitrary real Hermitian \({\mathcal P}{\mathcal T} \)-symmetric Hamiltonian, the reality of the ground-state energy for small enough coupling constant is established up to second order in perturbation theory.