What seems, on the surface, like pop’s oddest couple turns out to be a musical marriage made in heaven. Although there’s a 60-year age difference between Gaga and Tony, there’s real chemistry between the two on this inspired collection of American Songbook favorites. “I’ve been singing jazz standards since I was 13,” says Gaga, “but whatever authenticity you hear in my vocals is because of Tony. I can be myself around him.” The “Applause” singer sounds especially comfortable on “Nature Boy,” showing her smooth side over sensuous strings before Tony joins her on the flute-flecked Nat King Cole classic. Whether dueting on Cole Porter’s “Anything Goes” and Irving Berlin’s “Let’s Face the Music and Dance” or scatting their way through Duke Ellington’s “It Don’t Mean a Thing (If It Ain’t Got That Swing),” there’s a warmth and playfulness between them that’s entirely infectious. And their harmony is positively exquisite on the joyous title track.