Regulus took a few slow steps into the study, able to see the rows and columns of numbers that Draco was working on, though he couldn't make out anything specific. His cousin didn't look quite himself, maybe a little more frayed around the edges than Regulus was accustomed to. But then it was late. Regulus wasn't even sure of how late, was just aware of the fact that he certainly hadn't expected anyone to be up for the last few hours to see his return.
"So are you," Regulus returned easily, smiling slightly at the other. "Is this when you carry on all your secret affairs with numbers?" he asked, his tone lightly, trying to be teasing.