  We consider closed orientable hypersurfaces in a wide class of warped product
manifolds, which include space forms, deSitter-Schwarzschild and
Reissner-Nordstr\"{o}m manifolds. By using a new integral formula or Brendle's
Heintze-Karcher type inequality, we present some new characterizations of
umbilic hypersurfaces. These results can be viewed as generalizations of the
classical Jellet-Liebmann theorem and the Alexandrov theorem in Euclidean
space.
