えっ、そうなの？

E, sōnano?

Oh, is that so?

Word for word

“Eh? Is that so?”

Politeness: Casual

Plain form. Fine with family, close friends and people younger than you. Rude to a stranger or a boss.

When you would say it

Surprised, soft reaction to news.

Worth knowing

なの is the feminine-leaning but widely used casual question.