So if we interviewed in early September and haven't gotten an A are we just waiting for the R?

no i feel like they go back and accept people. but they MIGHT reject some people today. just a speculation based on last year...:)

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Does anyone know how often they release As?

once a month

Should I send an update/interest letter if I didn't receive an interview? Any advice in general?

They don't allow one.

They accept them post interview though right?