:In modern English—at least, especially in American English—it's just a typographical nicety.  One might as well say that dotless i is an English "letter" because typography sometimes undots an i (as in the fi-ligature ﬁ).  Most articles and titles don't bother to use æ except where invoking a brand name's characteristic typography such as Æon Flux or Encyclopædia Britannica; cf. Aeacus, Aeaea, Aebutius, etc.  It is a letter in some language's abecedaries, but Modern English is not one of them. (It may be the case that it remains a letter in words borrowed from a languages where it is a letter, e.g. Danish Æbelø or Old English æsc, but in most cases æ in English reflects Latin or Latinized Greek, where it is only a ligature of a and e.) — talk (la.wiktionary)