what do you think you are playing at, walking off into the forest with Viktor Krum? I want you to swear, by return owl, that you are not going to go walking with anyone else at night. There is somebody highly dangerous at Hogwarts.
I'm wondering if we should add an article in "by return owl", like "by a return owl". Why does it not take any articles here? How should we understand it?
Excerpted from Harry Potter