We propose to study the Infrared spectra of normal OB
 main sequence stars in order to investigate the structure of
 the outer layers of the photosphere and to determine the possible
 presence of a weak stellar wind. Ground-based observations of
 normal OB dwarfs demonstrate that IR Hydrogen lines (Br-alpha)
 can be strongly in emission while the H-alpha line is in absorption
 (Waters et al. 1993; Murdoch et al. 1994). Model calculations of
 the photospheric and wind emission of OB dwarfs indicate that, for
 very low mass loss rates, the Hydrogen recombination lines in
 the 4 to 40 micron wavelength range are still expected to show
 emission. This is due to the fact that the IR continuum of hot
 stars drops very rapidly with increasing wavelength, whereas
 the strength of the HI recombination lines decreases
 more slowly. This results in easily detectable
 emission lines in the 10-40 micron window.
 However in a recent paper by Murdoch el al. (1994)
 it is suggested that the IR emission may be due to a non-LTE
 effect in the outermost photospheric layers of hot stars. Both
 models predict different line strength ratios for the IR emission
 lines. Measurement of the strength of several Hydrogen lines in
 the SWS wavelength range will allow us to distinguish between
 both models. The study of the outer layers of the atmospheres
 of (hot) stars and of weak stellar winds can only be done with
 ISO.