The ring of $D_4$ triality invariants

Abstract

$D_4$ triality invariants are modular forms as well as invariant polynomials for a fiber product of the modular group and the Weyl group of type $F_4$. We show that the ring of $D_4$ triality invariants satisfying a certain cusp condition is isomorphic to the ring of joint covariants of a binary cubic and a binary quadratic form.