Romans type IIA theory and the heterotic strings

In this paper we study $T^2$ compactification of 6-dimensional massive type IIA supergravity in presence of Ramond-Ramond background fluxes. The resulting theory in four dimensions is shown to possess $SL(2,R)\times SL(2,R)\times O(4,20)$ duality symmetry. It is shown that specific elements of this symmetry relate massive type IIA compactified on $K3\times T^2$ (with fluxes along $K3$) to the ordinary type IIA compactified on $K3\times T^2$ (with fluxes along $T^2$). In turn, this relationship is exploited to relate Romans theory to heterotic strings. The D8-brane (domain-wall) wrapped on $K3\times T^2$ is related to {\it pure gravity} heterotic solution which is a direct product of 6-dimensional flat spacetime and a 4-dimensional Taub-NUT instanton.