  We present a quantitative study of the stellar and wind properties of massive
stars in the compact star forming region SMC-N81. Non-LTE spherically extended
atmosphere models including line-blanketing have been computed with the code
CMFGEN (Hillier & Miller 1998) and the analysis of UV STIS spectra lead to the
following results: 1) The SMC-N81 components are young (~ 0-4 Myrs) O stars
with Teff compatible with medium to late subtypes and with luminosities lower
than average Galactic O dwarfs, rendering them possible ZAMS candidates; 2) The
winds are extremely weak: the mass loss rates (~1e-8/1e-9 Msol/yr) and the
modified wind momenta are 1 to 2 orders of magnitude lower than observed so far
for Galactic stars, and are lower than the predictions of the most recent
hydrodynamical models. The reasons for such weak winds are investigated with
special emphasis on the modified wind momenta: 1) There may be a break-down of
the wind momentum - luminosity relation (WLR) for dwarf stars at low luminosity
(log L/Lsol < 5.5). However, reasons for such a breakdown remain unknown; 2)
The slope of the WLR may be steeper at low metallicity. This is predicted by
the radiation driven wind theory, but the current hydrodynamical simulations do
not show any change of the slope at SMC metallicity. Moreover, there are
indications that some Galactic objects have wind momenta similar to those of
the SMC stars; 3) Decoupling in the atmosphere may lead to multicomponent
winds, but various tests indicate that this is not likely to be the case for
the SMC-N81 stars. Hence, the origin of the weakness of the wind observed in
these objects remains unknown. We suggest that this weakness may be linked with
the youth of these stars and represents possibly the onset of stellar winds in
recently formed massive stars.
