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In this paper, we study the performance of a network comprised of a primary user and a secondary user with the latter having cognitive radio capabilities. The secondary node uses the empty slots of the primary user to transmit its own traffic as well as to relay the primary's traffic in a cooperative fashion. Taking a queuing theory approach, we find the probability generating functions of the numbers of packets in the queues of the primary and secondary users. Subsequently, we determine a number of performance measures such as the average queues' lengths, average packet transmission delays and secondary user's queue surcharge due to cooperation. The numerical results along with the simulations show the importance of controlling the number of primary user packets admitted by the secondary user for cooperation and its impacts on the other performance measures.