  Layered transition-metal dichalcogenides have attracted great interest in the
last few years. Thinned down to the monolayer limit they change from an
indirect band structure to a direct band gap in the visible region. Due to the
monolayer thickness the inversion symmetry of the crystal is broken and spin
and valley are coupled to each other. The degeneracy between the two equivalent
valleys, K and K', respectively, can be lifted by applying an external magnetic
field. Here, we present photoluminescence measurements of CVD-grown tungsten
disulphide (WS${_2}$) monolayers at temperatures of 2 K. By applying magnetic
fields up to 7 T in Faraday geometry, a splitting of the photoluminescence
peaks can be observed. The magnetic field dependence of the A-exciton, the
trion and three bound exciton states is discussed and the corresponding
g-factors are determined.
