Kelly Rowland & Paulina Rubio Signing on for 'The X Factor'

"The X Factor" is upping the sexy factor -- TMZ has learned Kelly Rowland and Paulina Rubio are signing on to be judges for next season.

Sources connected to FOX tell TMZ the pair are in final negotiations to join the show ... with only minor details left to be hammered out before they sign on.

We're told the deals will be done in the next few days, possibly as early as tomorrow.

Rowland and Rubio would be replacing Britney Spears and L.A. Reid ... and joining returning judges Demi Lovato and (of course) Simon Cowell.