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)

This star was only mentioned in writing.
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 and was located in the Beta Quadrant.