We present a compared analysis of some properties of indefinite almost
![$\mathcal{S}$](img14.png)
-manifolds
and indefinite
![$\mathcal{S}$](img14.png)
-manifolds. We give some characterizations in terms of the Levi-Civita connection and of the characteristic vector fields. We study the sectional and
![${\varphi}$](img15.png)
-sectional curvature of indefinite almost
![$\mathcal{S}$](img14.png)
-manifolds and state an expression of the curvature tensor field for the indefinite
![$\mathcal{S}$](img14.png)
-space forms. We analyse the sectional curvature of indefinite
![$\mathcal{S}$](img14.png)
-manifold in which the number of the spacelike characteristic vector fields is equal to that of the timelike characteristic vector fields.
Some examples are also described.