  We obtain new relations between Einstein-Yang-Mills (EYM) amplitudes
involving N gauge bosons plus a single graviton and pure Yang-Mills amplitudes
involving N gauge bosons plus one additional vector boson inserted in a way
typical for a gauge boson of a "spectator" group commuting with the group
associated to original N gauge bosons. We show that such EYM amplitudes satisfy
U(1) decoupling relations similar to Kleiss-Kuijf relations for Yang-Mills
amplitudes. We consider a D-brane embedding of EYM amplitudes in the framework
of disk amplitudes involving open and closed strings. A new set of monodromy
relations is derived for mixed open-closed amplitudes with one closed string
inserted on the disk world-sheet and a number of open strings at the boundary.
These relations allow expressing the latter in terms of pure open string
amplitudes and, in the field-theory limit, they yield the U(1) decoupling
relations for EYM amplitudes.
