(a)Retirement benefits. A retirement annuity payable to a former spouse under § 20.4 is adjusted for cost-of-living increases under section 826 or 858 of the Act in the same manner as the annuity of the principal. The first such increase for a former spouse shall be prorated under the applicable section in the same way the first increase for the principal is adjusted, irrespective of whether the annuity to the former spouse commences on the same date as the annuity to the principal. If the benefit of a former spouse is based in part on an annuity supplement payable to a principal under 5 U.S.C. 8421 which is not adjusted by COLA, then that portion of the benefit payable to a former spouse is not adjusted by COLA.(b)Survivor benefits.(1) Survivor annuities payable to a former spouse are adjusted for COLA under section 826 or 858 of the Act in the same manner as annuities are or would be adjusted for other survivors of the principal.(2) A survivor annuity payable to a former spouse under § 20.5-1(A) shall be increased from its commencing date pursuant to paragraph (c)(2) of section 826 of the Act or 8462 of Title 5, U.S. Code, by all COLA received by the principal at death, irrespective of the date of death and in instances where death occurred prior to December 22, 1987, by all COLA that would have been paid to a survivor annuitant from the date of death until December 22, 1987.(3) The first increase to which a former spouse becomes entitled whose annuity is computed under § 20.5(a)(2) shall be prorated pursuant to 5 U.S.C. 8462(c)(4) .(4) The first increase to which a former spouse becomes entitled whose annuity is computed under § 20.5(a)(3) or(5) Shall be prorated pursuant to paragraph (c)(1) of section 826 of the Act or 8462 or title 5, U.S. Code.