How sweet! Rihanna helped celebrate the release of Katy's new single, 'ROAR' last night at the Lion in New York City.

surprise appearanceJohn Mayer

Maybe Katy can return the love when Rihanna's second fashion collection for River Island launches next month. The 80-piece collection is described as a mix of "urban sports lux with sophisticated chic," and includes items like skirts, jumpers and a range of underwear.

"I literally designed everything I want to wear, exactly how I want to wear it," Rihanna said. "We're giving it a New York street vibe."

The Autumn/Winter 2013 collection launches a "first wave" on Sept. 12, with a "second wave" due in November.