From June 18:
According to Aftonbladet, Per surprised Princess Victoria and Daniel Westling with a performance of a brandnew song at yesterday’s party. Aftonbladet describes it as a romantic ballad specially written for the couple. But that’s not all, there’s another song to be revealed at the actual wedding party – where Roxette will perform on Saturday! We don’t know yet if any of these songs are to be released by Roxette or who will sing the song on Saturday. Stay tuned! 😀

Thanks abysmo for the newstip!

Update, June 28: As tevensso reports at TDR, “Per says he did not perform a song for the couple on Thursday.”

So either the “special song” story was totally made up or the song was presented without a live performance. Also the rumours about Roxette performing a new song at the wedding party on Saturday were not true.