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When several legacy information systems have to cooperate, the issue is to define a global framework providing both semantic interoperability and loose coupling among heterogeneous local information sources. To conciliate such requirements, we propose an object-oriented framework dedicated to the semantic evaluation of global queries while respecting local autonomy. Several layers insure interoperability through application objects and actual cooperation at a meta-level composed of federative objects. A unified description of local data allows structural and semantic evaluations during query processing. To preserve local autonomy, the description is dynamically derived from local data structures. Federative objects exploit a cooperation protocol to coordinate the exchanges during query evaluation.