  We address the question of why low-luminosity radio sources with similar flat
radio spectra show a range of optical activity. The investigation is based on
the spectral energy distributions (SEDs) of objects from the 200 mJy sample. We
gathered new data from the VLA at 43 GHz, from SCUBA in the JCMT at 2000, 1350
and 850 micron and from the ISOPHOT instrument on ISO at 170, 90, 60 and 25
micron. There is considerable diversity amongst the SEDs of the objects: there
are objects with steep broadband spectra between centimetre and millimetre
bands (14% of the sample), there are those with flat broadband spectra over
most of the spectral range (48% of the sample) and there are those which show
pronounced sub-mm/infrared excesses (27% of the sample). Some objects of the
first group have two sided radio morphology indicating that their pc-scale
emission is not dominated by beamed jet emission. Amongst the objects that have
smooth broadband spectra from the radio to the infrared there are passive
elliptical galaxies as well as the expected BL Lacs objects. The most
pronounced sub-mm/infrared excesses are shown by the broad emission line
objects.
