  We present a study of the magnetoresistance, the specific heat and the
magnetocaloric effect of equiatomic $RET$Mg intermetallics with $RE = {\rm
La}$, Eu, Gd, Yb and $T = {\rm Ag}$, Au and of GdAuIn. Depending on the
composition these compounds are paramagnetic ($RE = {\rm La}$, Yb) or they
order either ferro- or antiferromagnetically with transition temperatures
ranging from about 13 to 81 K. All of them are metallic, but the resistivity
varies over 3 orders of magnitude. The magnetic order causes a strong decrease
of the resistivity and around the ordering temperature we find pronounced
magnetoresistance effects. The magnetic ordering also leads to well-defined
anomalies in the specific heat. An analysis of the entropy change leads to the
conclusions that generally the magnetic transition can be described by an
ordering of localized $S=7/2$ moments arising from the half-filled $4f^7$
shells of Eu$^{2+}$ or Gd$^{3+}$. However, for GdAgMg we find clear evidence
for two phase transitions indicating that the magnetic ordering sets in
partially below about 125 K and is completed via an almost first-order
transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets
and rather pronounced for the ferromagnets for low magnetic fields around the
zero-field Curie temperature.

Materials Science