I'm looking for the title of a book series I read at as child.
The story starts on a future Earth in which people are sent back to live in medieval times. However, unknown to the future Earth, the old Earth is now ruled by aliens who capture all time travelers and use them as slaves by controlling them with collars. Bronze collars are for the working class, silver collars are for telepaths and gold collars are for humans collaborating with the aliens.
I never finished these books as a child and now it's been nagging me how they end.