Signia

In the Headmasters series Signia was the planet where Optimus Prime and a small crew of Autobots consisting of Arcee, Hot Rod, Kup and Jazz had set up a base. Its position in relation to Cybertron isn't known but in the Episode "The Tale of The Master Star" it is clearly the planet where The Autobots of Optimus Prime's crew meets with Fortress Maximus and the Autobot Headmasters whom have arrived from The Master Star.