The arrival roster at Deep Space 9 mentioning Deneb XX
The location of Deneb in "The Explored Galaxy" wall chart
Deneb XX was the twentieth planet in the Deneb planetary star system. This system was located in the Alpha Quadrant.
Deneb XX was visited by the GHD Per'ot in 2370 prior to the ship's arrival at Deep Space 9 on stardate 47566.8. (Star Trek VI: The Undiscovered Country, production art; DS9: "Whispers")
Deneb XX was never mentioned or shown; it was simply listed in an okudagram graphic of an arrival roster on a viewscreen. Deneb XX had the highest ordinal of the canon planets.
In "The Explored Galaxy" star chart, Deneb was located between the Tholian Assembly and Romulus. Both locations were identified in DS9: "The Search, Part II", "Call to Arms" as being located in the Alpha Quadrant. The star chart made the first chronological appearance in Star Trek VI: The Undiscovered Country.
According to Star Trek: Star Charts, there were two systems that shared the name Deneb. Deneb Kaitos system only had five planets, suggesting Deneb XX was located in the Alpha Cygni system. (pgs. 12-13, 41)