Sirius IV

Sirius IV was a planet colonized by Humans. According to the jurisprudence of the 26th century, however, it was not, legally, a colony.

By at least the year 2540, it had gained "Dominion status" with the Earth Empire. This gave it great autonomy within the Empire, and meant that it dealt with Earth mostly as an equal. The Master once used this special status as a way to negotiate for the release of the Doctor and Jo Grant into his custody. By fabricating charges against them on Sirius IV, he was able to successfully argue that he had greater claim to the prisoners than Earth Security. (DW: Frontier in Space)

According to the novelisation the planet was called Alderberan Four, and it had only recently become a Dominion.