  We report on broad-band electron spin resonance measurements performed within
the itinerant ferromagnetic phase of $R$CoPO ($R$ = La, Pr, Nd and Sm). We
reveal that the $R$ substitution is highly effective in gradually introducing a
sizeable easy-plane magnetocrystalline anisotropy within the Co sublattice. We
explain our results in terms of a subtle interplay of structural effects and of
indirect interactions between the $f$ and $d$ orbitals from $R$ and Co,
respectively.
