Covariance of WDVV equations

The (generalized) WDVV equations for the prepotentials in $2d$ topological and $4,5d$ Seiberg-Witten models are covariant with respect to non-linear transformations, described in terms of solutions of associated linear problem. Both time-variables and the prepotential change non-trivially, but period matrix (prepotential's second derivatives) remains intact.Comment: LaTeX, 7 pages, no figures (misprints corrected