Of course, the Doctor was always going to join River at the party--but all the more so, considering the news. Something he doesn't need to bring up, especially not in public, so instead he greets her by approaching from behind and stopping just at her side with the faintest of "incidental" nudges of his arm against hers and a smile cast toward her sidelong.

"Ah, good evening, Professor Song."

It's not hard for her to tell the more formal way of addressing her is put on for a bit of fun--much like his tuxedo. He hilts his head toward her, raising his eyebrows.

"Dance ticket's not full yet, I hope."