X Factor, Rihanna and L.A. Reid X Factor, Rihanna and L.A. Reid

Rihanna will be the "Only Girl (In the World)" when she mentors the boys group on The X Factor.

The "Umbrella" singer, 23, will be the guest mentor for judge L.A. Reid's group during the Judges' Homes round of the competition. During this round, each judge takes their group to their home or a location of their choosing to work with the contestants on a more intimate level and to also bring a second mentor to help them out.

Mariah Carey to mentor X Factor singers ... from Simon Cowell's crib?

After working with their judge and guest mentor, each contestant must then perform to prove why they deserve to be chosen for their judge's final category.  

Although it has been reported that Mariah Carey will serve as Simon Cowell's guest mentor, the other three guest mentors won't be officially revealed until Wednesday's show.

The X Factor airs Wednesdays and Thursdays at 8/7c on Fox.

Who else do you think should be a guest mentor for the contestants?