20 CFR 218.39: When a divorced spouse annuity ends.

A divorced spouse annuity ends with the earliest of the last day of the month before the month in which the—

(a) Divorced spouse dies;

(b) Employee's entitlement to an annuity ends;

(c) Divorced spouse marries;

(d) Employee dies; or

(e) Divorced spouse becomes entitled to a retirement or disability insurance benefit under the Social Security Act based on a primary insurance amount which equals or exceeds the amount of the full divorced spouse annuity before reduction for age.