Per IRS Publication 501, you must be "considered married" in order to file a joint return. Here's how the IRS defines that:
Considered married.
You are considered married for the whole year if, on the last day of your tax year, you and your spouse meet any one of the following tests.
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You are married and living together.
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You are living together in a common law marriage recognized in the state where you now live or in the state where the common law marriage began.
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You are married and living apart but not legally separated under a decree of divorce or separate maintenance.
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You are separated under an interlocutory (not final) decree of divorce.
https://www.irs.gov/pub/irs-pdf/p501.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
