20 CFR 219.50: When evidence of “living with” is required.

Evidence of “living with” (see part 222 of this chapter on Family Relationships) is required when—

(a) The employee's spouse applies for a spouse's annuity as a deemed spouse; or

(b) The employee's legal widow or widower applies for a lump-sum death payment, annuity payments due the employee but unpaid at death, or a residual lump-sum death payment on the basis of that relationship, or the employee's deemed widow or widower applies for a widow's or widower's annuity.