I wonder where the b in sein conjugation came from? are (v.) present plural indicative of be (q.v.), from Old English earun (Mercian), aron (Northumbrian),

I wonder where the b in sein conjugation came from? are (v.) present plural indicative of be (q.v.), from Old English earun (Mercian), aron (Northumbrian), from Proto-Germanic *ar-, probably a variant of PIE *es- "to be" (see am). Also from Old Norse cognates. https://www.etymonline.com/word/are https://www.etymonline.com/word/are