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The diaeresis or trema in French is used over the E only in the groups AE (Gaël) and OE (Moët), to indicate that the two vowels are not a diphtong and to differenciate these groups from the doble letters Æ and Œ. But in the group IE, there is never a trema over the E.

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So, Mariëtte is not French (in French, it would be a misspelling), the only correct French form is Mariette (pronounced [mari'et]).

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