F1: Did I tell you I got a letter from Bob last week?

F2:No, you didn't. How is he?

F1: pretty good. He is going to Brazil this summer.

F2: That sounds interesting. By the way, Did you hear that his old girlfriend is getting married?

F1:Really? you mean Amy?

F2: Yeah. To some guy she met at the beach last summer.

F1: Huh. Is that so. Oh, well.. Such is love....

Q: "Is that so." is a question. Why is not " Is that so?" ?


I would keep it with the period as it is not a real question, it is a statement expressing disapproval. (Imagine said with a downward sneery voice).
I have had many discussions over whether sentences such as: Will you please let me know when you arrive need a question mark. The form is a question, but you are not asking one really - will you is just a polite formula. Sometimes I put a question mark and sometimes I don't.
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I would have written it as a question. "Is that so?"
 nona the brit's reply was promoted to an answer.
 Forbes's reply was promoted to an answer.
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Hi, Nona,
Disapproval? I heard mild surprise there!