London: Music mogul Simon Cowell has reportedly signed on R&B star Rihanna to be a mentor on the US 'X Factor' to help judge L A Reid choose his final acts.

The 23-year-old singer is set to help Reid choose his final four acts after Cowell was so impressed with the 'Only Girl (In The World)' hitmaker's performance on the UK version of his talent show last year, reported a website.

 "Simon loves Rihanna and loved having her on the UK show last year - so it's no surprise he wants her back. He hasn't officially announced her yet as he loves keeping everyone guessing. But fans are already getting excited," a source

Rihanna was rumoured to be in the running for a place on the judging panel, but she did not make the final line-up - which comprises Reid, Simon, Paula Abdul and Nicole Scherzinger.

Singer Mariah Carey will also be acting as a guest mentor on the series.