We propose to study the nature of the coldest sources observed by IRAS. The
targets are only detected in the 100 micron IRAS band (with Fnu > 4 Jy in
the main field and Fnu > 1 Jy in the reference field) or have a F60/F100
flux ratio of less than 0.2 which is the typical colour of diffuse cirrus
clouds. We propose to obtain ISOPHOT 100 and 200 micron data of a sample
of these sources. The sample is selected from two pre-defined regions on
the sky.