  We present optical spectroscopy of the brightest 170 microns sources in the
FIRBACK South Marano (FSM) field. The survey is 90% complete at the 4 sigma
level. The spectra are compared with those of a reference sample of faint IRAS
(60 microns selected) sources observed in the same conditions . The galaxies in
both samples are characterized by a predominance of emission-line spectra and
moderate IR luminosities (10E10.5 < L_IR < 10E12 L_Sun). The fraction of AGN's
is low (about 15%) and the majority of sources are nearby (z<0.3), dusty, star
forming galaxies, with moderate star formation rates (a few 10 solar masses per
year). The infrared emission of the FSM galaxies shows also a colder spectral
energy distribution than was expected.
  The galaxies in both samples (IRAS faint sources and FSM) are essentially
undistinguishable with the available data, and seem to represent a population
of closeby, cold, star forming galaxies rather neglected up to now. Although
their contribution to the far-IR background seems to be low, they deserve a
more detailed study on their own. The contribution of fainter, presumably more
distant, galaxies to the far-IR background will be discussed when we will have
completed the follow-up of the fainter part of this 170 microns survey. It
seems however already clear that the fraction of closeby, cold galaxies is much
larger than expected by the earlier models, at least down to the sensitivity
achieved by this ISO survey.
