G. Lagache , Jean-Loup Puget and the FIRBACK collaboration
Institut d'Astrophysique Spatiale, Orsay, France
We have conducted a deep cosmological survey with the ISOPHOT instrument
at 175 
in several high latitude areas relatively free
of cirrus contamination (see the presentation by Dole et al.) in search
of the objects responible for the recently discovered Cosmic Far InfraRed
Background (CFIRB).
Our survey finds a large population of point sources. The main, but striking, result is that number counts are considerably in excess of those expected for the local FIR population discovered by IRAS. Nevertheless, these counts are consistent with predictions of the source population responsible for the CFIRB. The detected sources have a much higher 175/60 color ratio than the IRAS galaxies. These sources are thus either a new population of nearby very cold galaxies or, more likely, more distant sources (with typical redshift around 1.5). We suggest that these sources are probably the bright end of the population responsible for the background. A substantial multiwavelenghts follow up program is under way to determine the source population.