This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
While tracking the whereabouts of the Klingon vessel that kidnapped Gabriel Lorca, Rhys charted three possible courses, all passing near the Mempa system, as well as the nearby Morska system and Rura Penthe. (DIS: "Choose Your Pain")
In 2256, this star's location was labeled on the star chart "Alpha/Beta Quadrant Overview" in the ready room aboard the USS Discovery. The star was located in the territory of the Klingon Empire. (DIS: "Magic to Make the Sanest Man Go Mad", "The War Without, The War Within")
The system was the site of several key battles during the Klingon Civil War of 2367-2368. Forces loyal to Gowron suffered a major defeat against the House of Duras in the Mempa system during the conflict. The battle ended decisively in the Duras' favor, and Gowron's forces were sent into full retreat. Sela assured B'Etor that Gowron's forces would not be a threat in the region for some time. (TNG: "Redemption II")
However, Martok's bold leadership on that day kept the battle from turning into a complete rout. Miles O'Brien, posing as a Klingon named Pahash, claimed to the Martok Changeling to have fought at Mempa. Ironically, in a way he had "fought" with Martok, since he had served on the USS Enterprise-D, which was nearby during the Klingon Civil War, supported Gowron, and ultimately aided in exposing the Duras family. (DS9: "Apocalypse Rising") Martok, who was not entirely convinced, recalling Mempa, stating that "many noble deeds were done on that dark day." (DS9: "Apocalypse Rising")