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We point out that kinetic and St\"uckelberg mixings that are genericallypresent in the low energy effective action of axions can significantly widenthe window of axion decay constants. We show that an effective super-Planckiandecay constant can be obtained even when the axion kinetic matrix has onlysub-Planckian entries. Our minimal model involves only two axions, aSt\"uckelberg U(1) and a modest rank instanton generating non-Abelian group.Below the mass of the St\"uckelberg U(1), there is only a single axion with anon-perturbatively generated potential. In contrast to previous approaches, theenhancement of the axion decay constant is not tied to the number of degrees offreedom introduced. We also discuss how kinetic mixings can lower the decayconstant to the desired axion dark matter window. String theory embeddings ofthis scenario and their phenomenological features are briefly discussed.

Widening the Axion Window via Kinetic and St\"uckelberg Mixings