The Romulan system was an inhabited planetary system in Sector Z-6 of the Beta Quadrant. The system consisted of a primary and at least four planets. The second and third planets were Romulus and Remus, the homeworlds of the Romulans and Remans, respectively. (TOS: "Balance of Terror", display graphic; Star Trek VI: The Undiscovered Country, production art; DS9: "The Search, Part II"; Star Trek Nemesis)
During the Neutral Zone Incursion of 2266, the location of the Romulan system was labeled on a star chart of Sector Z-6 that was displayed on the bridge viewscreen of the USS Enterprise. (TOS: "Balance of Terror", production art)
In 2293, the location of Romulus and Remus in the Milky Way Galaxy was labeled in a star chart that was in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
According to Star Trek: Star Charts (p. 66) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"; "Position of the Romulan Empire"), the Romulus system was located in the Beta Quadrant. It was a binary system with a M-class star and a K-class star. This system was in the Romulan War Theater of Operations (2156-2160). In 2266, the Praetor's flagship departed the Romulus system on its mission to the Neutral Zone. According to a map of the Romulan Empire – prepared by the Stellar Archive Artist Lsel Essik, from the Romulan Master Data Catalog , in 2366 – Romulus was identified as a Romulan star.
According to Star Trek Maps (p. 29) and The Worlds of the Federation (p. 148), the Romulan star system was a binary star system of two white dwarfs known by the Federation as Romulus (primary) and Romulus B (secondary). In Diane Duane's Rihannsu novels, the Romulan system was a single G9-class star, known as 128 Trianguli to the Federation and Eisn to the Romulans ("homesun" in the Romulan language).