A rational group of hermitian type is an algebraic group over the rational
numbers whose symmetric space is a hermitian symmetric space. We assume such a
group G to be given, which we assume is isotropic. Then, for any rational
parabolic P in the group G, we find a reductive rational subgroup N
closely related with P by a relation we call incidence. This has implications
to the geometry of arithmetic quotients of the symmetric space by arithmetic
subgroups of G, in the sense that N defines a subvariety on such an
arithmetic quotient which has special behaviour at the cusp corresponding to
the parabolic with which N is incident.Comment: 29 pages (11 pt), ps-file also available at the home page
http://www.mathematik.uni-kl.de/~wwwagag, preprints. LaTeX v2.0