Who is RAB in Harry Potter?

Previously mentioned in the interview quoted above, Anelli suggested to Rowling that R.A.B. could stand for Regulus [A.] Black. Regulus, the younger brother of Harry's godfather Sirius Black, has been mentioned in passing but has not had any plot significance yet.[3] The following conversation ensued:

Rowling: "Well, I think that would be, umm, a fine guess.".

Melissa: "Forgive me if I'm remembering incorrectly, but was Regulus the one who was murdered by Voldemort?"

Rowling: "Well Sirius said he wouldn't have been because he wasn't important enough, remember?"

Melissa: "But that doesn’t have to be true, if [R.A.B.] is writing Voldemort a personal note."

Rowling: "That doesn't necessarily show that Voldemort killed him, personally, but Sirius himself suspected that Regulus got in a little too deep. Like Draco. He was attracted to it, but the reality of what it meant was way too much to handle."

Rowling had already been asked about Regulus prior to the publication of The Half-Blood Prince, during World Book Day online chat, March 4, 2004. Rowling elusively answered the question "Will we be hearing anything from Sirius Black's brother, Regulus, in future books?" with "Well, he's dead, so he's pretty quiet these days."

I guess we'll just find out when we read the book. =P

When someone says Rob in a Scottish accent, it sounds like Rab. I don't know otherwise.

Well, the general consensus is Regulus (Arthurus?) Black, Sirius' brother.

Regulus A. Black

Regulus A. Black, I think he is either Sirius's cousin or brother

i'm not sure but i guess we'll find out in the next book...i can't wait for it to come out