  Using a large sample of charm semileptonic decays collected by the FOCUS
photoproduction experiment at Fermilab, we present evidence for a small, even
spin K- \pi+ amplitude that interferes with the dominant K*0 component in the
K-pi+ mu- nu final state. Although this interference significantly distorts the
decay angular distributions, the new amplitude creates only a very small
distortion to the observed kaon pion mass distribution when integrated over the
other kinematic variables describing the decay. Our data can be described by
K*0 interference with either a constant amplitude or broad spin zero resonance.
