  We prove a Ramanujan-type formula for $520/\pi$ conjectured by Sun. Our proof
begins with a hypergeometric representation of the relevant double series,
which relies on a recent generating function for Legendre polynomials by Wan
and Zudilin. After showing that appropriate modular parameters can be
introduced, we then apply standard techniques, going back to Ramanujan, for
establishing series for $1/\pi$.
