  F-theory, as a 12-dimensional theory that is a contender of the Theory of
Everything, should be compactified into elliptically fibered threefolds or
fourfolds of Calabi-Yau. Such manifolds have an elliptic curve as a fiber, and
their bases may have singularities. We considered orbifold as simplest non-flat
construction. Blow up modes of orbifold singularities can be considered as
coordinates of complexified Kahler moduli space. Quiver diagrams are used for
discribing D-branes near orbifold point. In this case it is possible to
calculate Euler character defined through $\mbox{Ext}^i(A,B)$ groups and
coherent sheaves $A, B$ over projective space, which are representations of
orbifold space after blowing up procedure. These fractional sheaves are
characterized by $Q_0$, $Q_2$ and $Q_4$ Ramon-Ramon charges, which have special
type, calculated for $C^3/Z_3$ case. BPS central charge for $C^3/Z_3$ orbifold
is calculated through Ramon-Ramon charges and Picard-Fuchs periods.
