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Published online by Cambridge University Press: 20 November 2018
Let $f$ be a classical newform of weight 2 on the upper half-plane ${{H}^{(2)}}$, $E$ the corresponding strong Weil curve, $K$ a class number one imaginary quadratic field, and $F$ the base change of $f$ to $K$. Under a mild hypothesis on the pair $(\,f\,,\,K)$, we prove that the period ratio ${{\Omega }_{E}}/(\sqrt{\left| D \right|}{{\Omega }_{F}})$ is in $\mathbb{Q}$. Here ${{\Omega }_{F}}$ is the unique minimal positive period of $F$, and ${{\Omega }_{E}}$ the area of $E(\mathbb{C})$. The claim is a specialization to base change forms of a conjecture proposed and numerically verified by Cremona and Whitley.