What does your lease say? Generally both tenants would still be obligated to pay the rent through the end of the lease but not after that. Does your lease automatically convert to month to month, or do you need to sign a new contract? Does the remaining tenant meet the income requirements? If not, you need to serve a 30 day notice that they are to be out (this is a little late by now) or let them know they can find a new roommate that meets the requirement and passes the background checks.
Moving forward, I would start making the tenants figure this out sooner- I generally like to have new leases signed at least 2 months prior to the old ones expiring, and generally ask for tenants to decide by 3 months before the lease end date. This works great in my market but may not in yours. I offer incentives for signing the new lease on my timeline- if they sign by my date the rent is XX, if they do not and I begin marketing the rent goes up to whatever it would be for a new tenant. It is worth it to me for $25 or even $50 per month less rent to have current tenants stay.
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