Twin modular S4 with SU(5) GUT

We discuss the SU(5) grand unified extension of flavour models with multiple modular symmetries. The proposed model involves two modular S-4 groups, one acting in the charged fermion sector, associated with a modulus field value tau(T) with residual Z(3)(T) symmetry, and one acting in the right-handed neutrino sector, associated with another modulus field value tau(SU) with residual Z(2)(SU) symmetry. Quark and lepton mass hierarchies are naturally generated with the help of weightons, which are SM singlet fields, where their non-zero modular weights play the role of Froggatt-Nielsen charges. The model predicts TM1 lepton mixing, and neutrinoless double beta decay at rates close to the sensitivity of current and future experiments, for both normal and inverted orderings, with suppressed corrections from charged lepton mixing due to the triangular form of its Yukawa matrix.