Abrupt cutoffs in the optical-infrared spectra of nonthermal sources

Optical and infrared photometry is reported for a number of radio sources with very faint and red optical identifications. Three types of object are indicated for the sample: highly redshifted galaxies with approximately normal colors; sources subjected to strong Galactic reddening; and violently variable sources with very sharp intrinsic spectral cutoffs toward higher frequencies. The spectra of the latter class cut off nearly as abruptly as is theoretically possible for synchrotron emission.