  Heavy flavour decay to light hadrons is the key to understanding many aspects
of the SM from CP violation to strong dynamics. It is often presumed in line
with the simple quark spectator model of D decay to Kpipi that the Kpi system
has only I=1/2. E791 have recently presented an analysis of their results on D+
to (K-pi+)pi+ using a generalised isobar picture of two body interactions.
While higher Kpi waves are described by sums of known resonances, the S-wave
amplitude and phase are determined bin-by-bin in Kpi mass. The phase variation
is found not to be that of K-pi+ elastic scattering. This hints at a different
mixture of I=1/2 and I=3/2 S-wave interactions than in elastic scattering.
Applying Watson's theorem to this generalised isobar model allows us to
estimate the I=3/2 Kpi S-wave component. We indeed find that this is larger
than in hadronic scattering or semileptonic processes.
