  The transferred hyperfine interaction between nuclear and electron spins in
an heavy fermion material depends on the hybridization between the $f$-electron
orbitals and those surrounding a distant nucleus. In CeMIn$_5$ (M=Rh, Ir, Co),
both the hyperfine coupling to the two indium sites as well as the crystalline
electric field at the Ce are strongly dependent on the transition metal. We
measure a series of CeRh$_{1-x}$Ir$_x$In$_5$ crystals and find that the
hyperfine coupling reflects the orbital anisotropy of the ground state Ce 4$f$
wavefunction. These findings provide direct proof that the localized to
itinerant transition is dominated by hybridization out of the Ce-In plane in
this system.
