The Tau Cygna system was a planetary system that was located in the de Laure belt, over five thousand light years from the Sol sector. According to Section 133, Paragraph 77 of the Treaty of Armens, the fifth planet had been ceded by the Federation to the Sheliak Corporate in 2255.
In the late 23rd century, the colony ship SS Artemis landed on Tau Cygna V, the only planet in range suitable to do so. The survivors built a colony which lasted for nearly a century. However, under pressure from the Sheliak Corporate, the colonists were relocated to a new home. (TNG: "The Ensigns of Command")