Theta functions for Holomorphic triples

We introduce an generalization of the theta divisor to the theory of holomorphic triples on a smooth projective curve $X$. We show that a given triple $T=(E_1\to E_0)$ is $\alpha$-semistable iff there exists an orthogonal tripe $S=(F_1\to F_0)$ with given numerical invariants. This yields globally generated theta line bundles on the moduli space of semistable triples.