The De Divortio ("Concerning Divorce") is an extended mid ninth-century treatise written by Hincmar, Archbishop of Reims (d. 882), which survives in a single manuscript, Paris BnF. lat. 2866. It explores the issues arising from the attempt by Lothar II, king of Lotharingia (855-869), to rid himself of his wife Teutberga and replace her with his concubine, Waldrada. Hincmar is primarily concerned with defining what marriage is and how (or if) it may be ended, and with the duties of bishops and of kings. However, in the course of discussing these questions, he touches on many other issues too, and gives much detail on ninth-century politics and religious practice in Frankia.
Editions
The most recent edition is that of L. Böhringer, De Divortio Lotharii Regis Et Theutbergae Reginae (1992) in the Monumenta Germaniae Historica series. A provisional English-language translation can be found at http://hincmar.blogspot.com.