Lease Violation - Potential Legal Issues?

[Rich C.]

I came to find out that my tenants have acquired a "long term guest".  One roommate just left for several months and the long term guest has now moved into their room and pays them a portion of rent.  This new person also has a dog.  This is all a violation of the lease because it was done without my knowledge and approval.  I am wondering if I need to be concerned legally that this person is living there and not on the lease?