  We study the post-merger mass ejection of low-mass binary neutron stars (NSs)
with the system mass of $2.5\, M_\odot$, and subsequent nucleosynthesis by
performing general-relativistic, neutrino-radiation viscous-hydrodynamics
simulations in axial symmetry. We find that the merger remnants are long-lived
massive NSs surviving more than several seconds, irrespective of the nuclear
equations of state (EOSs) adopted. The ejecta masses of our fiducial models are
$\sim 0.06$-$0.1\, M_\odot$ (depending on the EOS), being $\sim 30\%$ of the
initial disk masses ($\sim 0.15$-$0.3\, M_\odot$). Post-processing
nucleosynthesis calculations indicate that the ejecta is composed mainly of
light $r$-process nuclei with small amounts of lanthanides (mass fraction $\sim
0.002$-$0.004$) and heavier species due to the modest average electron fraction
($\sim 0.32$-$0.34$) for a reasonable value of the viscous coefficient. Such
abundance distributions are incompatible with the solar $r$-process-like
abundance patterns found in all measured $r$-process-enhanced metal-poor stars.
Therefore, low-mass binary NS mergers should be rare. If such low-mass NS
mergers occur, their electromagnetic counterparts, kilonovae, will be
characterized by an early bright blue emission because of the large ejecta mass
as well as the small lanthanide fraction. We also show, however, that if the
effective turbulent viscosity is very high, or there is an efficient mass
ejection working in the early post-merger phase, the electron fraction of the
ejecta could be low enough that the solar $r$-process-like abundance pattern is
reproduced and the lanthanide fraction becomes so high that the kilonova would
be characterized by early bright blue and late bright red emissions.
