katy perry and john mayer are making beautiful music together. the celebrity couple went public in a song for john s new album. it s risky business of course to mix work and play. oh, yeah. trust me. but they say the music made them do it. we are up all nightline with abc s david wright. my boy ain t the one reporter: a loving duet debuted on gma. that bucking bull the only hint of the turmoil of celebrity romance. katy is a superstar for a reason. aww! reporter: in real life, katy perry and john mayer may have had their ups and downs but as they told george stephanopoulos, they re emphatically back together. the two of you never sat down for a tv interview together? yep. we don t quite know exactly what kind of body language we are supposed to be exhibiting. reporter: for such public figures it is not easy. too much touchy-feely or too