  We explore two classes of 6d $\mathcal{N}=(1,0)$ little string theories
obtained from type IIA/IIB NS5-branes probing $D_n$ singularities. Their tensor
branches are described by effective gauge theories whose instanton solitons are
macroscopic little strings. We specifically study two families of 2d
$\mathcal{N}=(0,4)$ gauge theories which describe at low energy the worldsheet
dynamics of the type IIA/IIB little strings. These gauge theories are useful to
calculate the supersymmetric partition functions of the little string theories
on $\mathbf{R}^4 \times T^2$. We establish the T-duality of the little string
theories by utilizing their BPS spectra as a probe.
