Orbifolds versus smooth heterotic compactifications

Following the recent exploration of smooth heterotic compactifications with unitary bundles, orbifold compactifications in six dimensions can be shown to correspond in the blow-up to compactifications with U(1) gauge backgrounds. A powerful tool is the comparison of anomaly polynomials. The presentation here focuses on heterotic SO(32) compactifications in six dimensions including five-branes. Four dimensional and E8 x E8 models are briefly commented on.