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OTRC: Taylor Swift and Justin Bieber wrote a song together?

Taylor Swift appears in a scene from her music video 'Sparks Fly,' which was released on Aug. 10, 2011. / Justin Bieber appears in an undated photo from his official website, JustinBieberMusic.com. (Big Machine Records Inc. / JustinBieberMusic.com)

Taylor Swift caused a frenzy this week when she had Justin Bieber join her on-stage at her concert and now the pop stars are collaborating on music together, according to their producer friend Kuk Harrell.

"Wow!! Got to see @justinbieber & @taylorswift13 write a song together last night!! SMASH!!!! SMASH!!! SMASH!!! Let's do it again Yall!!!" Harrell wrote on Twitter on Saturday, which Bieber re-Tweeted.

On Friday evening, Bieber cryptically wrote, "#writing."

Thaddis "Kuk" Harrell is a songwriter and vocal producer who has worked with Jennifer Lopez, Beyonce, Mary J. Blige and Rihanna. He has won five Grammy Awards for his involvement in Beyonce's "Single Ladies (Put a Ring on It)," Mary J. Blige's "Growing Pains" and Rihanna's "Umbrella," which he co-wrote.

On August 23, Justin Bieber surprised Swift's fans by jumping on-stage to sing his hit, "Baby" with Swift, during her concert at Los Angeles' Staples Center.

"You think you've heard LOUD screaming in your life... Then @justinbieber comes out and does a surprise song during your show. Whoah. Unreal," Swift wrote on Twitter, following the performance.

Bieber returned the favor, posting a video (watch below) from their duet with the caption, "@Taylorswift13 has been there from day 1. true friend. #goodtimes."

The 21-year-old Grammy-winning country and pop singer has wowed audiences this summer with her romance-themed U.S. concerts that include stunning costumes, ballet dancers, pyrotechnics, high-wire aerialists and a wedding scene.

In July of this year, Swift's "Speak Now" tour, aimed at promoting the album, sold more tickets via vendor TicketsNow than any other music artist, bumping McCartney, 69, to second place.

The singer recently released the single, "Sparks Fly," which is the fifth single from "Speak Now," Swift's third studio album. The record has sold more than 3 million copies in the United States since its October 2010 release.

The singer recently added 10 Australia and New Zealand shows to her "Speak Now" world tour. The singer's U.S. tour dates are largely sold-old through late November. Check out Swift's tour dates on her official website.

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