Sigma Nelvana was the primary of the Nelvana system. The star was located within the Romulan Neutral Zone.
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)
The location of Sigma Nelvana on a tactical intelligence analysis star chart
This star was only mentioned in writing.