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We present a class of exactly solvable quantum spin models which consist oftwo Heisenberg-subsystems coupled via a long-range Lieb-Mattis interaction. Thetotal system is exactly solvable whenever the individual subsystems aresolvable and allows to study the effects of frustration. We consider (i) theantiferromagnetic linear chain and (ii) the Lieb-Mattis antiferromagnet for thesubsystem-Hamiltonians and present (i) the complete ground-state phase diagramand (ii) the full thermodynamic phase diagram. We find a novel phase whichexhibits {\it order from disorder} phenomena.Comment: 4 pages, latex, accepted for publication in Europhysics Letters,  figures available from voigt@voigt.nat.uni-magdeburg.d

Integrable Models of Interacting Quantum Spins with Competing Interactions