more proof that R.A.B. is regulus black
courtesy of wikipedia:
There are no characters specifically mentioned in the books who have exactly R.A.B. as initials, apart from those identified by Hermione. There are some who have incomplete names, or whose initials might be correct if they customarily use their second name, or have a changed surname (perhaps by marriage). However, there is further evidence for the identity of R.A.B. from foreign language editions of the book. These contain various alternative names for the principal characters, as well as the initials from the note. For example, the Dutch edition uses the initials R.A.Z. The Norwegian edition has R.A.S. The Finnish edition has the letters R.A.M. The French, Portuguese, Spanish and Italian editions retain the initials R.A.B..
Translation of the Harry Potter books into authorized editions in other languages are performed by authors other than J. K. Rowling herself, although she would undoubtedly have approval and veto authority over crucial plot details. Nevertheless these alternate editions may not be considered absolutely canonical. The author is not fluent in many of the languages and cultures that the series is printed in, and therefore must relinquish much literary license to the translators.
There are no characters specifically mentioned in the books who have exactly R.A.B. as initials, apart from those identified by Hermione. There are some who have incomplete names, or whose initials might be correct if they customarily use their second name, or have a changed surname (perhaps by marriage). However, there is further evidence for the identity of R.A.B. from foreign language editions of the book. These contain various alternative names for the principal characters, as well as the initials from the note. For example, the Dutch edition uses the initials R.A.Z. The Norwegian edition has R.A.S. The Finnish edition has the letters R.A.M. The French, Portuguese, Spanish and Italian editions retain the initials R.A.B..
Translation of the Harry Potter books into authorized editions in other languages are performed by authors other than J. K. Rowling herself, although she would undoubtedly have approval and veto authority over crucial plot details. Nevertheless these alternate editions may not be considered absolutely canonical. The author is not fluent in many of the languages and cultures that the series is printed in, and therefore must relinquish much literary license to the translators.



Comments on this journal
HAHAHAH those are MY initials :)
The character who has had the greatest support as a candidate to be R.A.B. is Regulus Black, the younger brother of Sirius Black. Previously mentioned in the interview quoted above, Melissa suggested to Rowling that R.A.B. could stand for Regulus A. Black. The following conversation ensued:
Rowling: "Well, I think that would be, um, 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 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."
Many believe these comments to be indirect confirmation that R.A.B. is Regulus Black.
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 answering the question: "Will we be hearing anything from Sirius Black's brother, Regulus, in future books?" said: "Well, he's dead, so he's pretty quiet these days."
Regulus is mentioned three times in The Half-Blood Prince, by Albus Dumbledore, by Horace Slughorn, and then by Remus Lupin, early in the book. In comparison, his brother Sirius was mentioned by Hagrid in the opening chapter of the first book, with no further mention until The Prisoner of Azkaban, where he was the title character alongside Harry.
Regulus was a Death Eater who tried to disassociate himself from Voldemort. He was killed on Voldemort's orders (Sirius notes that Regulus was too unimportant to warrant Voldemort doing this himself). The note left by R.A.B. addresses Voldemort as The Dark Lord, and Death Eaters are one of the only groups who customarily refer to him thus.
Regulus' middle initial is unknown (it does not appear on the family tree tapestry at 12 Grimmauld Place, the Black family home and headquarters of The Order of the Phoenix). However it is possible he may have been given the middle name Alphard (after his uncle), or Arcturus (after his grandfather), as those names do appear on the Black Family Tree. The family tree was derived from a hand-drawn sketch of the tree made by J.K. Rowling herself, and from information extracted from the novels and various interviews.