My dictionary says seance. Wikipedia says séance.

At what point does it become acceptable to spell a word adopted from another language without the accented characters?

I don't know, I'm just posing the question.

It's kind of weird that we keep the accents on any foreign words at all when they're imported into English. After all, English doesn't have accents; at least kids don't learn about them in school the same way that French kids presumably learn about accented characters when they learn to read and write. When we import words from Japanese or Arabic or whatever, we don't leave them in the glyphs of the original language - we transcribe them in to the best approximation we can with our standard 26 letters.

There's the obvious argument that an accent is only a minor change from the letters we're familiar with as English speakers, and in some cases it can help distinguish between the imported word and another word that would otherwise be spelt the same.

I'm not coming down on either side here. I'm just pointing out that it's a bit weird that some foreign words retain accents while others don't.