(written from a Production point of view)
In the script of the Star Trek: Deep Space Nine episode "Profit and Loss", Sadera VI is a planet. According to StarTrek.com, this planet was located in the Sadera system, which was located between Cardassia and Bajor. ; 
In a scene in act two in the script, Natima Lang is thinking of ways to get her students Hogue and Rekelen safely out of the reach of the Cardassian Central Command and thinks about taking them to other Cardassian dissident movement members on Sadera VI. (Star Trek: Deep Space Nine Companion - A Series Guide and Script Library)
The Star Trek: Terok Nor novel Dawn of the Eagles mentions the Sadera system in the context of it being a refuge for the Cardassian dissident movement. In the novel, Gul Russol tries to convince Lang to go there.