The Dopa system was an inhabited planetary system located inside Cardassian space in the Alpha Quadrant.
Following the discovery of the attack on Korma by a Klingon Bird-of-Prey commanded by K'Temang in 2372, Kira Nerys inquired as to the distance of the nearest Cardassian warship; Gul Dukat guessed that the closest warship was probably somewhere in the Dopa system, a three day journey from Korma.
Later, when Dukat laid a trap for the same Bird-of-Prey, he claimed that his freighter, the Groumall, was carrying replicator and transporter components to the Dopa system. (DS9: "Return to Grace")
This system was only mentioned in dialogue.
This system's quadrant of origin is inferred based on the position of the Cardassian Union as seen in the star chart appearing in the Star Trek: Picard episode "Maps and Legends".
The Dopa system was identified as a binary star system, which had a G-class star and F-class star, by the Star Trek: Star Charts (p. 46) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385").