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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.