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