i think R.A.B is Seriuses brother! if u think about it, it make's sence!
I don't think so. Come on people, you should all know by now that J.K. Rowling does not go for the obvious. Everyone thought that the Half-Blood Prince was Harry or Voldemort or Hagrid, no one guessed Snape. For all we know, R.A.B could be two people or even a group. We need to start thinking outside the box.
I agree that Regalus would be too obvious, but as for it being more than one person...the note found in the locket used the word "I" like 6 or 7 times. Wouldn't the note have said "we" instead. No I definately think it is one person. Who, I don't know yet, but just one. That's just my opinion though.
I think RAB might be regulus and something related to his death, something that (we) Harry will have to find out in the next book, or
RAB can be something related with the private lessons that Dumbledore (GOD Bless his soul) was giving Harry, what is to say, with Tom's past, before he becoming a Dark Lord, or
RAB is nothing, that Dumbledore invented to keep Tom Riddle bussy, and give Harry time to find the other pieces of Voldermort's soul.
[PHP]To the Dark Lord, I know I will be dead long before you read this but I want you to know that it was I who discovered your secret. I have stolen the real Horcrux and intend to destroy it as soon as I can. I face death in hope that when you meet your match, you will be mortal once more R.A.B.[/PHP] But who says that isn't a bad thing, that this R.A.B. has it?