Zac Efron: hair gel user and a cappella crooner. (Kevin Winter/Getty Images)

As long as celebrities on late-night talk show hosts are willing to cooperate, we at Celebritology are happy to start off your mornings with clips of famous people singing.

Yesterday, we brought you Bruce Springsteen and Jimmy Fallon/Neil Young doing “Sexy and I Know It.” Today, we’ve got Zac Efron, Conan O’Brien, and “Conan” band leader La Bamba doing three-part harmony on a classic Beach Boys hit.

During his appearance Monday on “Conan,” Efron — whose voice currently stars in “The Lorax” and whose condom recently made an accidental cameo appearance on a red carpet — launched into a clearly pre-planned story about how he and his buddies once did an impromptu a cappella performance in Paris.

“A ton of girls loved it,” Efron noted, while O’Brien expressed surprise that girls would get all swoony over a bunch of young, handsome boys singing in unison. (Mr. O’Brien, we know you went to Harvard. And we know Harvard had at least one a cappella group during your time there. Surely you know that females are total suckers for this stuff.)

Anyway, the conversation eventually led — as most conversations do — to a live a cappella performance of “Barbara Ann,” with O’Brien doing the high part. Watch and listen (the crooning begins around 2:22), then feel free to get nostalgic and commence Facebook-searching for that super-cute guy from your college’s a cappella group who sang lead on “Uptown Girl” back in 1993 and now works as a Web developer who hums incessantly while coding.