  We find an optimal upper bound on the volume of the John ellipsoid of a
$k$-dimensional section of the $n$-dimensional cube, and an optimal lower bound
on the volume of the L\"owner ellipsoid of a projection of the $n$-dimensional
cross-polytope onto a $k$-dimensional subspace. We use these results to give a
new proof of Ball's upper bound on the volume of a $k$-dimensional section of
the hypercube, and of Barthe's lower bound on the volume of a projection of the
$n$-dimensional cross-polytope onto a $k$-dimensional subspace. We settle
equality cases in these inequalities.
  Also, we describe all possible vectors in $\R^n,$ whose coordinates are the
squared lengths of a projection of the standard basis in $\R^n$ onto a
$k$-dimensional subspace.
