Sigma Nelvana
Sigma Nelvana was the primary of the Nelvana system. This Beta Quadrant system was located within the Romulan Neutral Zone.
Sigma Nelvana and its five planets
In 2366, the location of Sigma Nelvana was labeled on both a Starfleet Tactical intelligence analysis star chart and system map. (TNG: "The Defector", okudagram)
This star was only mentioned in writing.
This star's quadrant of origin is inferred based on the position of its star system as seen in the star chart appearing in the Star Trek: Picard episode "Maps and Legends".
According to Star Trek: Star Charts (p. 66) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Sigma Nelvana was a M-class star.