The geometry of bi-Perron numbers with real or unimodular Galois conjugates

Among all bi-Perron numbers, we characterise those all of whose Galois conjugates are real or unimodular as the ones that admit a power which is the stretch factor of a pseudo-Anosov homeomorphism arising from Thurston's construction. This is in turn equivalent to admitting a power which is the spectral radius of a bipartite Coxeter transformation.