The arrival roster at Deep Space 9 mentioning Deneb XX
The location of Deneb in "The Explored Galaxy" star chart
Deneb XX was the twentieth planet in the Deneb 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 has only been mentioned in the arrival roster okudagram. Deneb XX has the highest ordinal of the canon planets. In "The Explored Galaxy" star chart, Deneb was located between the Tholian Assembly and Romulus. Both were identified in DS9: "Call to Arms" as being located in the Alpha Quadrant.
According to Star Trek: Star Charts (pgs. 12-13, 41), 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.