At the Billie Eilish concert in California, Charli xcx made a guest appearance. It's not the first time the artists have celebrated birthdays together. Billie Eilish turned 23 on December 18th.

Billie Eilish's latest concert got the Brat treatment. On Tuesday, Dec. 17, Charli xcx made a surprise appearance at the "Birds of a Feather" hitmaker's Los Angeles concert at the Kia Forum.

On Tuesday, Charli XCX celebrated the end of her monumental ... Charli also included a photo with Billie Eilish from their panty party music video for the remix of “Guess.” ...

Credit: Terrance O'Connor As well as achieving huge success with her own album, Hit Me Hard And Soft, Billie Eilish also scored a big hit this year with her guest spot on the remix of Charli XCX ...

Billie Eilish helps inaugurate Billboard's new Hot Dance/Pop Songs chart with her first top 10 on the tally, "Guess" with ...

Charli XCX, Kim Kardashian and more celebs spotted Billie Eilish, Charli XCX at the Hit Me Hard and Soft Tour at Kia Forum on Dec. 17. Christopher Polk/Billboard/Getty Images The stars were out in ...

Charli XCX may have dominated this year but there were a whole load of blockbuster records following in its wake ...

Charli XCX isn’t your typical pop star ... the songwriting savant finally got her due this summer (IYKYK). Billie Eilish builds on “Happier Than Ever” with “Hit Me Hard and Soft.” ...

By Gil Kaufman Billie Eilish celebrated her 23rd birthday a day early on Tuesday (Dec. 17) at the Kia Forum in Los Angeles when she invited Charli XCX up on stage for a surprise collaboration on ...