20 CFR §404.344

You may be eligible for benefits if you are related to the insured person as a wife, husband, widow, or widower. To decide your relationship to the insured, we look first to State laws. The State laws that we use are discussed in § 404.345. If your relationship cannot be established under State law, you may still be eligible for benefits if your relationship as the insured's wife, husband, widow, or widower is based upon a deemed valid marriage as described in § 404.346.