  The UV and optical spectra from the HST FOS and from the FAST spectrograph at
the Whipple Observatory at different phases are analysed, leading to new
results about the configuration of AG Peg. Consistent modelling by the code
SUMA, which accounts for the coupled effect of photoionisation from the hot
star and the shock, shows the important role of at least three nebulae: 1) the
nebula between the stars, downstream of the shock created by collision of the
winds, which propagates in reverse towards the WD, 2) the nebula downstream of
the shock expanding in the outskirts of the giant atmosphere, and 3) the shock
propagating outwards the binary system, that is not reached by the hot source
radiation. The relative contribution of the three nebulae to the final spectra
at different phases are calculated for all the lines. It is found that the
contribution to the different lines changes with the viewing angle of the
system. The relatively low preshock density and magnetic field adopted in the
modelling of the expanding shocks reveal that they are merging with the ISM,
thus explaining the slow decline of AG Peg.
