  We show that two interacting physical systems may admit entangled pure or non
separable mixed states evolving in time as if the mutual interaction
hamiltonian were absent. In this paper we define these states Interaction Free
Evolving (IFE) states and characterize their existence for a generic binary
system described by a time independent Hamiltonian. A comparison between IFE
subspace and the decoherence free subspace is reported. The set of all pure IFE
states is explicitly constructed for a non homogeneous spin star system model.
