20 C.F.R. § 222.14
If a ceremonial or common-law marriage relationship cannot be established under State law, a claimant may still be found to have the relationship as spouse of an employee based upon a deemed marriage. A claimant is deemed to be the wife, husband, or widow(er) of the employee if the person's marriage to the employee would have been valid under State law except for a legal impediment, and all of the following requirements are met:
[54 FR 42949, Oct. 19, 1989, as amended at 65 FR 20726, Apr. 18, 2000]