Romans type IIA theory and the heterotic strings

In this Letter we study T2 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×T2 (fluxes along K3) to the ordinary type IIA compactified on K3×T2 (fluxes along T2). In turn, this relationship is exploited to relate Romans theory to heterotic strings. The D8-brane (domain-wall) wrapped on K3×T2 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.