Published online by Cambridge University Press: 01 September 2014
We use an invariant-theoretic method to compute certain twists of the modular curves $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}X(n)$ for $n=7,11$. Searching for rational points on these twists enables us to find non-trivial pairs of $n$-congruent elliptic curves over ${\mathbb{Q}}$, that is, pairs of non-isogenous elliptic curves over ${\mathbb{Q}}$ whose $n$-torsion subgroups are isomorphic as Galois modules. We also find a non-trivial pair of 11-congruent elliptic curves over ${\mathbb{Q}}(T)$, and hence give an explicit infinite family of non-trivial pairs of 11-congruent elliptic curves over ${\mathbb{Q}}$.