  We present Keck spectroscopy and UKIRT near-IR imaging observations of two
170micron-selected sources from the ISO-FIRBACK survey which have faint
counterparts in the optical, and r-K~5. Both sources were expected to lie at
z>1 based on their far-infrared, submillimeter and radio fluxes, assuming a
similar spectral energy distribution to the local ultra-luminous infrared
galaxy (ULIRG) Arp220. However, our spectroscopy indicates that the redshifts
of these galaxies are z<1: z=0.91 for FN1-64 and z=0.45 for FN1-40. While the
bolometric luminosities of both galaxies are similar to Arp220, it appears that
the dust emission in these systems has a characteristic temperature of 30K much
cooler than the ~50K seen in Arp220. Neither optical spectrum shows evidence of
AGN activity. If these galaxies are characteristic of the optically faint
FIRBACK population, then evolutionary models of the far-infrared background
must include a substantial population of cold, luminous galaxies. These
galaxies provide an important intermediate comparison between the local
luminous IR galaxies, and the high redshift submillimeter-selected galaxies,
for which there is very little information available.
