Japanese: あ、そう。[a sou]
Gernman: Ach so!
English: Oh / Is it!
The response [a sou] is used in almost same situations
in Japanese and in German.
That is, you can use [a sou] in various situations.
For example, you can imply
"I am not interested in your story" or
"Wow. I had no idea about that!"
depending on the intonation.
