Who's Britney's Celebrity Guest Judge?

It better be a superstar!
Each X Factor judge is paired up with a fellow-celeb for an episode to help the contestants grow into a better performer.
We’ve learned the judges will receive an age group that best fits their persona, and now we know two of the four celebrity guest judges.
LA Reid is working with Justin Bieber in the 25+ age division, and Demi Lovato is paired up with Nick Jonas for the 12-16 group.
So who’s Britney’s working with?!