  We report the analysis of time-resolved spectroscopy of IP Pegasi in outburst
with eclipse mapping techniques to investigate the location and geometry of the
observed spiral structures. We were able to obtain an improved view of the
spiral structures with the aid of light curves extracted in velocity bins
matching the observed range of velocities of the spiral arms combined with a
double default map tailored for reconstruction of asymmetric structures.
Two-armed spiral structures are clearly seen in all eclipse maps. The arms are
located at different distances from the disc centre. The ``blue'' arm is
farther out in the disc (R= 0.55 +/- 0.05 R_{L1}) than the ``red'' arm (R= 0.30
+/- 0.05 R_{L1}). There are evidences that the velocity of the emitting gas
along the spiral pattern is lower than the Keplerian velocity for the same disc
radius. The discrepancy is smaller in the outer arm (measured velocities 10-15
per cent lower than Keplerian) and is more significant in the inner arm
(observed velocities up to 40 per cent lower than Keplerian). We measured the
opening angle of the spirals from the azimuthal intensity distribution of the
eclipse maps to be \phi= 25 +/- 3 degrees. A comparison with similar
measurements on data at different outburst stages reveals that the opening
angle of the spiral arms in IP Peg decreases while the outbursting accretion
disc cools and shrinks, in agreement with the expected evolution of a tidally
driven spiral wave. The sub-Keplerian velocities along the spiral pattern and
the clear correlation between the opening angle of the spirals and the outburst
stage favors the interpretation of these asymmetric structures as
tidally-induced spiral shocks.
