Rihanna is to join The Voice US

Image source, PA

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.

Image source, Reuters

Image caption, Pharrell Williams performs in Denmark

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

Image source, AP

Image caption, 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.