Taylor Swift, Harry Styles Credit: Steve Granitz/WireImage.com; Kevin Mazur/WireImage.com

From American royalty (Conor Kennedy) to a newly-minted boy band prince! The new Us Weekly, out now, has brand-new details on Taylor Swift's flirt-fest with One Direction's Harry Styles at an X Factor performance rehearsal in L.A. Nov. 15.

"They lit up when they saw each other," an observer tells Us of freshly single Swift, 22, and the British crooner, 18. Spending much of the day together, Swift "was a puppy dog," the observer marvels of the "I Knew You Were Trouble" singer; Styles watched her rehearse while sitting with her mom, and even sang along!

After walking hand-in-hand on set, the duo made a dramatic, romantic exit: "Harry picked her up and carried her to the trailer!"

And it wasn't the first day of interaction for Styles and Swift, who split with Conor Kennedy back in October. "They email a lot!" a pal says.

For much more on Swift's latest romantic prospect, pick up the new Us Weekly, on stands now!