There are no extremal eutactic stars other than root systems

Abstract A eutactic star on an integral lattice is called extremal if it induces a holomorphic Jacobi form of lattice index and singular weight via the theta block. The famous Macdonald identities imply that root systems are extremal as eutactic stars. In this paper, we prove that every extremal eutactic star arises as a root system. This answers a question posed by Skoruppa.