  We demonstrate that a supersymmetric theory twisted on a K\"ahler four
manifold $M=\Sigma_1 \times \Sigma_2 ,$ where $\Sigma_{1,2}$ are 2D Riemann
surfaces, possesses a "left-moving" conformal stress tensor on $\Sigma_1$
($\Sigma_2$) in the BRST cohomology. The central charge of the Virasoro algebra
has a purely geometric origin and is proportional to the Euler characteristic
of the $\Sigma_2$ ($\Sigma_1$) surface. This structure is shown to be invariant
under renormalization group. We also give a representation of the algebra
$W_{1+\infty}$ in terms of a free chiral supermultiplet.
