Dimensional reduction of 4d heterotic string black holes

We perform the spherical symmetric dimensional reduction $4d\to 2d$ of heterotic string theory. We find a class of two-dimensional (2d) dilaton gravity models that gives a general description of the near-horizon, near-extremal behavior of four-dimensional (4d) heterotic string black holes. We show that the duality group of the 4d theory is realized in two dimensions in terms of Weyl transformations of the metric. We use the 2d dilaton gravity theory to compute the statistical entropy of the near-extremal 4d, $a=1\mathrm{/}\sqrt{3}$, black hole.