  Given number fields $L \supset K$, smooth projective curves $C$ defined over
$L$ and $B$ defined over $K$, and a non-constant $L$-morphism $h \colon C \to
B_L$,we consider the curve $C_h$ defined over $K$ whose $K$-rational points
parametrize the $L$-rational points on $C$ whose images under $h$ are defined
over $K$. Our construction provides a framework which includes as a special
case that used in Elliptic Curve Chabauty techniques and their higher genus
versions. The set $C_h(K)$ can be infinite only when $C$ has genus at most 1;
we analyze completely the case when $C$ has genus 1.
