===================================================================== ==> In this proposal, more time is being requested for RWATERS.OBDWARF ===================================================================== We propose to obtain high S/N spectra of selected hydrogen lines in hot OB dwarfs in order to probe the outer regions of their atmosphere and the region in which the onset of the radiation driven wind occurs. These outer regions are very difficult to study with other observing techniques because lines are often saturated and are formed over a very wide range of depths. We have already obtained ISO spectra of some stars, and detected emission in several, demonstrating the success of the approach. Here we ask for more observing time in order to complete the programme, which was severely compromised by the reduced in-orbit sensitivities of ISO, resulting in a present sample which is statistically not significant. With these data we will determine the mass loss rate and its dependence on stellar rotation rate.