  Different scenarios of new physics beyond the standard model give rise to a
direct coupling of right-handed quarks to W bosons. We will discuss how the
prediction of the Callan-Treiman low energy theorem for the scalar K pi form
factor in combination with recent data on $K^L_{\mu 3}$ decays can serve as a
stringent test of the standard model (SM) and possible extensions, in
particular right-handed charged quark currents (RHCs). In addition, we will
comment on the impact of hadronic tau decay data on RHCs.
