The feminine ordinal indicator (ª) and the masculine ordinal indicator (º) can be regarded as letters, too, since they correspond to letters "a" and "o" in specific situations.
The following characters are regarded as
independent
letters, although some of them are historically combinations of
two letters or a letter and a diacritic:
Æ æ (letter ae)
Ð ð (eth)
Þ þ (thorn)
Ø ø (o with stroke)
ß (sharp s)
Notice that the following characters are not regarded as
letters, despite being historically formed from one or more letters:
¢
£
¥
©
®
µ