Romans type IIA theory and the heterotic strings

In this paper we study T 2 compactification of six-dimensional massive type IIA supergravity in presence of Ramond-Ramond background fluxes. The resulting theory in four dimensions is shown to possess SL(2, R)×SL(2, R)×O(4, 20) duality symmetry. It is shown that specific elements of this symmetry relate massive type IIA compactified on K3 × T 2 (fluxes along K3) to the ordinary type IIA compactified on K3×T 2 (fluxes along T ) . In turn, this relationship is exploited to relate Romans theory to heterotic strings. The D8-brane (domain-wall) wrapped on K3 × T 2 is found to correspond to pure gravity heterotic solution which is a direct product of six-dimensional flat space and a four-dimensional Taub-NUT instanton.