The structure of : preliminariesRemember that should take values in some representation of the bosonic group, so in constructing the covariant exressions we should take into account the structure of .
We assume 2 things about :
The quadratic Casimir of vanishes (this is the difference of the two Casimirs)
Exists a vector invariant under
These two conditions are actually quite restrictive. The first condition implies that contained in the decomposition of functions on . The second condition implies that it is actually contained in the harmonic functions.
Basically, what we are doing is starting from the dilaton/axion waves corresponding to , and then reconstructing other linearized solutions by acting with the SUSY transformations, as e.g. in (13)