  We have investigated the electronic structure of the equiatomic EuAuMg,
GdAuMg, YbAuMg and GdAgMg intermetallics using x-ray photoelectron
spectroscopy. The spectra revealed that the Yb and Eu are divalent while the Gd
is trivalent. The spectral weight in the vicinity of the Fermi level is
dominated by the mix of Mg $s$, Au/Ag $sp$ and $RE$ $spd$ bands, and not by the
$RE$ $4f$. We also found that the Au and Ag $d$ bands are extraordinarily
narrow, as if the noble metal atoms were impurities submerged in a low density
$sp$ metal host. The experimental results were compared with band structure
calculations, and we found good agreement provided that the spin-orbit
interaction in the Au an Ag $d$ bands is included and correlation effects in an
open $4f$ shell are accounted for using the local density approximation +
Hubbard $U$ scheme. Nevertheless, limitations of such a mean-field scheme to
explain excitation spectra are also evident.
