Rihanna is to join The Voice US

Singer Rihanna is to join The Voice US as a key adviser to all the coaches.

It's about to start it's ninth series in the US on 21 September.

Pharrell Williams performs in Denmark

Pharrell Williams, Adam Levine from Maroon 5 and Blake Shelton will also return as mentors.

Gwen Stefani is returning as coach of NBC"s "The Voice" for its ninth season.

Singer Gwen Stefani will also be back - she previously served as a coach on season seven.