  In this study we report the results of study of novel ternary $Np_2PtGa_3$
compound. The x-ray-powder diffraction analysis reveals that the compound
crystallizes in the orthorhombic CeCu$_2$-type crystal structure (space group
Imma) with lattice parameters $a$ = 0.4409(2) nm, $b$ = 0.7077(3) nm and $c$ =
0.7683(3) nm at room temperature. The measurements of dc magnetization,
specific heat and electron transport properties in the temperature range 1.7 -
300 K and in magnetic fields up to 9 T imply that this intermetallic compound
belongs to a class of ferromagnetic Kondo systems. The Curie temperature of
$T_C \sim$ 26 K is determined from the magnetization and specific heat data. An
enhanced coefficient of the electronic specific heat of $\gamma$ = 180 mJ/(mol
at. Np K$^2$) and -ln\T dependence of the electrical resistivity indicate the
presence of Kondo effect, which can be described in terms of the S = 1
underscreened Kondo-lattice model. The estimated Kondo temperature $T_K \sim$
24 K, Hall mobility of $\sim$ 16.8 cm$^2$/Vs and effective mass of $\sim$ 83
$m_e$ are consistent with assumption that the heavy-fermion state develops in
$Np_2PtGa_3$ at low temperatures. We compare the observed properties of
$Np_2PtGa_3$ to that found in $Np_2PtGa_3$ and discuss their difference in
regard to change in the exchange interaction between the conduction and
localized 5f electrons. We have used the Fermi wave vector $k_F$ to evaluate
the Rudermann-Kittel-Kasuya-Yosida (RKKY) exchange. Based on experimental data
of the (U, Np)$_2$(Pd, Pt)Ga$_3$ compounds we suggest that the evolution of the
magnetic ground states in these actinide compounds can be explained within the
RKKY formalism.
