On the spectra of sl^(N)k/sl^(N)k\hat{sl}(N)_k/\hat{sl}(N)_k-cosets and WNW_N gravities

We study the spectra of G/G coset models by computing BRST cohomology of affine Lie algebras with coefficients in tensor product of two modules. One-to-one correspondence between the spectra of $A_1^1/A_1^1$ and that of the minimal matter coupled to gravity (including boundary states of the Kac table) is observed. This phenomena is discussed from the point of hamiltonian reduction of BRST complexes of $A_N^1$ Lie algebras. (In the revised version one proof is modified and examplified, some misprints are corrected.)Comment: 16 page

Theory of K\"ahler Gravity

In this paper we discuss the connection on a space of $N=2$ TCFT's that appears in the context of background (in)dependence. We formulate a family of {\it target space field theories} with a similar connection on it. Each theory is a gauge theory (with the gauge group being ${\cal SD}iff$ in the case of $3$-fold). It describes deformations of K\"ahler structures much like Kodaira Spencer theory describes deformations of the complex structures. It is manifestly background independent. It appears to be a target space field theory for supersymmetric quantum mechanics

Quantum cohomology of partial flag manifolds

We compute the quantum cohomology rings of the partial flag manifolds F_{n_1\cdots n_k}=U(n)/(U(n_1)\times \cdots \times U(n_k)). The inductive computation uses the idea of Givental and Kim. Also we define a notion of the vertical quantum cohomology ring of the algebraic bundle. For the flag bundle F_{n_1\cdots n_k}(E) associated with the vector bundle E this ring is found.Comment: 33 page

On Four-Dimensional Compactifications of F-Theory

Branches of moduli space of F-theory in four dimensions are investigated. The transition between two branches is described as a 3-brane-instanton transition on a 7-brane. A dual heterotic picture of the transition is presented and the F-theory - heterotic theory map is given. The F-theory data - complex structure of the Calabi-Yau fourfold and the instanton bundle on the 7-brane is mapped to the heterotic bundle on the elliptic Calabi-Yau threefold CY_3. The full moduli space has a web structure which is also found in the moduli space of semi-stable bundles on $CY_3$. Matter content of the four-dimensional theory is discussed in both F-theory and heterotic theory descriptions.Comment: 40 pages, TeX, references added, minor changes in Section