  Given a finite index subgroup of $SL_2(\mathbb Z)$ with modular curve defined
over $\mathbb Q$, under the assumption that the space of weight $k$ ($ \ge 2$)
cusp forms is $1$-dimensional, we show that a form in this space with Fourier
coefficients in $\mathbb Q$ has bounded denominators if and only if it is a
congruence modular form.
