Heterotic flux backgrounds and their IIA duals

We study four-dimensional heterotic flux vacua with N=2 spacetime supersymmetry. A worldsheet perspective is used to clarify quantization conditions associated to the fluxes and the constraints these place on the moduli spaces of resulting compactifications. We propose that these vacua fit naturally in the context of heterotic/IIA duality as heterotic duals to compactifications on K3-fibered but not elliptically fibered Calabi-Yau three-folds. We present some examples of such potential dual pairs.
Comment: 46 pages; corrected a quantization mistake in section 4.1 as well as a few typos; removed section 4.3 (due to further quantization errors)