Getty Images
"They're trying to keep it a secret," Dunham's "pal" told Us ... So it makes total sense that the friend went and told a celebrity gossip magazine about it.
"They have a good time when they can get together!" the source added.
Dunham, 26, and Antonoff, 28, are apparently mixing business with pleasure. The pair recently collaborated on a video for The New Yorker together, and according to the insider, "Lena's crazy about the band," and "has asked to use Fun. songs on 'Girls.'"
Jack and his band are up for two awards at tomorrow night's (Sept. 6) MTV Video Music Awards. Fun. is nominated for Best New Artist and Best Pop Video for their breakout hit, "We Are Young."
Watch "Fun. Band Members Talk About Their Origins"
Taylor Swift Dominates at 2013 Billboard Music Awards
Tim McGraw's 'Superstar Summer Night' Special to Air May 19
Tim McGraw Stops Atlanta Show to Help Ailing Fan
Carrie Underwood 'Goats' Hunter Hayes With Blown Away Tour Prank
Jennifer Hudson Performs 'Inseparable' With 'American Idol' Winner Candice Glover
Blake Shelton Will Return for Season 5 of 'The Voice'
Miguel and Fan Are 'Okay' After Crazy Kick at 2013 Billboard Music Awards
Stone Temple Pilots Tap Linkin Park's Chester Bennington as New Singer
Beyonce Releases New Anthem, 'Grown Woman'
Trace Adkins Named Winner of 'All-Star Celebrity Apprentice'