  Dual Pfaff equations (of the form \tilde D^a = 0, \tilde D^a some vector
fields of degree -1) preserved by the exceptional infinite-dimensional simple
Lie superalgebras ksle(5|10), vle(3|6) and mb(3|8) are constructed, yielding an
intrinsic geometric definition of these algebras. This leads to conditions on
the vector fields, which are solved explicitly. Expressions for preserved
differential form equations (Pfaff equations), brackets (similar to contact
brackets) and tensor modules are written down. The analogous construction for
the contact superalgebra k(1|m) (a.k.a. the centerless N=m superconformal
algebra) is reviewed.
