FRAP 28J
https://www.law.cornell.edu/rules/frap/rule_28
(j) Citation of Supplemental Authorities. If pertinent and significant authorities come to a party's attention after the party's brief has been filed—or after oral argument but before decision—a party may promptly advise the circuit clerk by letter, with a copy to all other parties, setting forth the citations. The letter must state the reasons for the supplemental citations, referring either to the page of the brief or to a point argued orally. The body of the letter must not exceed 350 words. Any response must be made promptly and must be similarly limited.