They were relaxed with each other, regularly proffering mutual admiration. Yet they still made an odd pair: the ultra-skinny Mr. Armstrong, 41, wearing a cropped leather jacket with his black hair in a punkabilly updo of tangled curls, and Ms. Jones, 34, a dark-eyed, round-faced girl next door wearing a casual sweater.
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Mr. Armstrong, who lives in Oakland, Calif., came to New York to make the album with Ms. Jones, who lives in Brooklyn. They didn’t tell their record companies. They booked five days in May — between Green Day tour dates — at the Magic Shop, a studio in SoHo, and returned for four days in August, for a total of nine days of recording with a rhythm section Ms. Jones chose the drummer Dan Riser and the bassist Tim Luntz.

“Foreverly” is the kind of side project that pop stars can do on a whim, schedules permitting. It doesn’t announce a new direction for Ms. Jones or Mr. Armstrong, except in one aspect, the spontaneity: “I don’t think I’ll ever make a record and tell anybody again,” Mr. Armstrong said. “It’s just better that way.”