  We give a new proof, inspired by an argument of Atiyah, Bott and Patodi, of
the first fundamental theorem of invariant theory for the orthosymplectic super
group. We treat in a similar way the case of the periplectic super group.
Lastly, the same method is used to explain the fact that Sergeev's super
Pfaffian, an invariant for the special orthosymplectic super group, is
polynomial.
