We present Keck spectroscopy and UKIRT near-IR imaging observations of two 170 \mu m-selected sources from the ISO -FIRBACK survey which have faint counterparts in the optical , and r - K \sim 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 ) Arp 220 .
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 Arp 220 , it appears that the dust emission in these systems has a characteristic temperature of \sim 30 K , much cooler than the \sim 50 K seen in Arp 220 .
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 .