  We present low temperature magneto-photoluminescence experiments which
demonstrate the brightening of dark excitons by an in-plane magnetic field $B$
applied to monolayers of different semiconducting transition metal
dichalcogenides. For both WSe$_2$ and WS$_2$ monolayers, the dark exciton
emission is observed at $\sim$50 meV below the bright exciton peak and displays
a characteristic doublet structure which intensity is growing with $B^2$, while
no magnetic field induced emission peaks appear for MoSe$_2$ monolayer. Our
experiments also show that the MoS$_2$ monolayer has a dark exciton ground
state with a dark-bright exciton splitting energy of $\sim$100 meV.
