We compute the genus one correction to the integrable hierarchy describing
coupling to gravity of a 2D topological field theory. The bihamiltonian
structure of the hierarchy is given by a classical W-algebra; we compute the
central charge of this algebra. We also express the generating function of
elliptic Gromov - Witten invariants via tau-function of the isomonodromy
deformation problem arising in the theory of WDVV equations of associativity.Comment: 53 pages, Latex, two references added, some typos corrected, version
to appear in Commun. Math. Phy
