"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"