The Romulan system was an inhabited planetary system. This system consisted of a primary and four planets. Two of the planets were inhabited. The first planet was suitable for colonization by humanoid lifeforms. Over many centuries, an offshot species of the Vulcans would find their way to this planet and they would found settlements on this planet. The colonists named their world Romulus. The other world, named Remus, was the homeworld to a pre-warp capable species, the Remans. (TOS: "Balance of Terror", Star Trek Nemesis)
Remus was conquered by the Romulans who conscripted the native population into slavery, and had them mine the dilithium deposits and build heavy weapons. Eventually, Romulus became the capital of an interstellar empire, the Romulan Star Empire. The system was located in Sector Z-6, a region of space in the Alpha Quadrant. The Romulan Neutral Zone was located near this system. (TOS: "Balance of Terror", display graphic; Star Trek VI: The Undiscovered Country, production art; DS9: "The Search, Part II"; Star Trek Nemesis)
The location of the system in the Milky Way Galaxy was depicted on a Federation star chart, in 2293. (Star Trek VI: The Undiscovered Country, production art)
In 2387, this system was devastated by the Supernova of 2387. At least one planet, Romulus, was destroyed. (Star Trek)
See also
Background information
According to Startrek.com, the primary star in the system was called Romulus. According to Star Trek Maps (page 29) and The Worlds of the Federation (page 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 indigenous name for the Romulan sun was Eisn.
According to the Star Trek: Star Charts (pg. 66), this system was located in or near the Romulan War Theater of Operations (2156-2160).
"The Explored Galaxy" star map was first seen chronologically in Star Trek VI: The Undiscovered Country. It was also seen in several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes set in the 24th century, from the year 2364 to 2370. These were TNG: "Conspiracy", "The Measure Of A Man", "The Emissary", "The Mind's Eye", "The Game", and DS9: "In the Hands of the Prophets" and "Cardassians".