In 1986, a system of equations for compactifications of
the heterotic string which preserve supersymmetry was proposed independently by C. Hull and A. Strominger.
They are more complicated than the
Calabi-Yau compactifications proposed earlier by P. Candelas, G. Horowitz, A. Strominger, and E. Witten, because they
allow non-vanishing torsion and they incorporate terms which are quadratic in the
curvature tensor. As such they are also particularly interesting from the point of
view of both non-Kaehler geometry and the theory of non-linear partial differential equations.
While the complete solution of such partial differential equations seems out of reach at the present time, we describe progress in developing a new general approach based on geometric flows which shares some features
with the Ricci flow. In particular, this approach can recover the non-perturbative solutions found in 2006 by J.X. Fu and S.T. Yau.