Fans of 'Smile' hitmaker Lily Allen finally have reason to, well, smile, as the once-retired U.K. singer returns with a duet on Pink's upcoming sixth album, 'The Truth About Love,' a star-studded affair that also features Eminem.

Titled 'True Love,' the song finds Pink trading verses with the Allen, who's recently been holed up in the studio working on her follow-up to 2009's 'It's Not Me, It's You,' which went Top Five on the Billboard charts and spawned five singles, including 'The Fear,' 'Not Fair' and '22.'

In recent years, Allen has turned away from music, choosing to focus on other areas of her life and career -- including launching her own label, opening a London clothing boutique, and having a baby with her husband, Sam Cooper. Over the summer, she announced she was working with producer Greg Kurstin on some new songs -- a series of sessions that presumably made time for Allen's collaboration with Pink.

The list of guest stars for 'The Truth About Love' doesn't end with Eminem and Allen, either; the NME reports that fun.'s Nate Ruess will also make an appearance, lending his vocals to a track titled 'Just Give Me a Reason.' The new LP is scheduled to arrive in stores on Sept. 18.

Listen to Pink and Lily Allen's 'True Love'