20 C.F.R. § 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.