HELP!! Is this legal? Do I have to assume these leases?

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I went under contract on a duplex on May 3rd.  There were tenants in the property that I was aware of but it was my understanding they were on Month to Month leases.  I am 1 week away from closing and I have been presented with leases on both sides of the duplex.  They are TWO year leases at way under market rent.  The leases were signed and executed on May 6th which is AFTER we went under contract.  Do I have to assume these leases?  What should I do?