  We show in a precise group theoretical fashion how the generating solution of
regular BPS black holes of N=8 supergravity, which is known to be a solution
also of a simpler N=2 STU model truncation, can be characterized as NS-NS or
R-R charged according to the way the corresponding STU model is embedded in the
original N=8 theory. Of particular interest is the class of embeddings which
yield regular BPS black hole solutions carrying only R-R charge and whose
microscopic description can possibly be given in terms of bound states of
D-branes only. The microscopic interpretation of the bosonic fields in this
class of STU models relies on the solvable Lie algebra (SLA) method. In the
present article we improve this mathematical technique in order to provide two
distinct descriptions for type IIA and type IIB theories and an algebraic
characterization of S*T--dual embeddings within the N=8,d=4 theory. This
analysis will be applied to the particular example of a four parameter
(dilatonic) solution of which both the full macroscopic and microscopic
descriptions will be worked out.
