  Low temperature and polarization resolved magneto-photoluminescence
experiments are used to investigate the properties of dark excitons and dark
trions in a monolayer of WS$_2$ encapsulated in hexagonal BN (hBN). We find
that this system is an $n$-type doped semiconductor and that dark trions
dominate the emission spectrum. In line with previous studies on WSe$_2$, we
identify the Coulomb exchange interaction coupled neutral dark and grey
excitons through their polarization properties, while an analogous effect is
not observed for dark trions. Applying the magnetic field in both perpendicular
and parallel configurations with respect to the monolayer plane, we determine
the g-factor of dark trions to be $g\sim$-8.6. Their decay rate is close to 0.5
ns, more than 2 orders of magnitude longer than that of bright excitons.
