  The new general upper bound mu <= [c/24] + 1 for the minimal weight mu of a
self-dual vertex operator superalgebra of central charge c different from 47/2
is proven. For central charges c <= 48, further improved estimates are given
and examples of with large minimal weight are discussed. We also study the case
of vertex operator superalgebras with N=1 supersymmetry which was first
considered by Witten in connection with three-dimensional quantum gravity. The
upper bound mu^* <= (1/2)[c/12]+1/2 for the minimal superconformal weight is
obtained for c different from 47/2.
  In addition, we show that it is impossible that the monster sporadic group
acts on an extremal self-dual N=1 supersymmetric vertex operator superalgebra
of central charge 48 in a way proposed by Witten if certain standard
assumptions about orbifold constructions hold. The same statement holds for
extremal self-dual vertex operator algebras of central charge 48.
