  A complete treatment of the entanglement of two-level systems, which evolves
through the contact with a thermal bath, must include the fact that the system
and the bath are not fully separable. Therefore, quantum coherent
superpositions of system and bath states, which are almost never fully included
in theoretical models, are invariably present when an entangled state is
prepared experimentally. We show their importance for the time evolution of the
entanglement of two qubits coupled to independent baths. In addition, our
treatment is able to handle slow and low-temperature thermal baths.
