Herr, herr! And especially since frau is accepted.
If
frau wasn't already accepted, I would have great doubts about admitting it. Almost every English dictionary I consulted had both
Herr and
Frau capitalised, as forms of address.
However, there is one exception. Wiktionary, has a listing for
frau, in lower case, but only
Herr, with a capital
H. The illustrative quote for
frau uses the word as a descriptive term rather than as a form of address:
It presents Dürer’s mother as a sharp-nosed, world-weary German frau looking wryly out at life from under a decorous wimple.
I don't think the word
herr is ever used in English in a similar way. It's only used preceding someone's surname, where it's normally written with a capital
H.
