We consider unitary evolution of finite bipartite quantum systems and study time dependence of purity for initial cat states -- coherent superpositions of Gaussian wave-packets. We derive explicit formula for purity in systems with nonzero time averaged coupling, a typical situation for systems where an uncoupled part of the Hamiltonian is Liouville integrable. Previous analytical studies have been limited to harmonic oscillator systems but with our theory we are able to derive analytical

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... s for general integrable systems. Two regimes are clearly identified, at short times purity decays due to decoherence whereas at longer times it decays because of relaxation.