  We investigate the nature of seven unusual radio galaxies from the 5C
catalogue that were previously known to have extremely red R-K colours, and for
which emission lines were previously found to be weak or absent in their
optical spectra. We present and discuss u, g, or r images of these radio
galaxies, obtained using the Optical System for Imaging and
low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at the Gran
Telescopio Canarias (GTC). We have detected all seven targets in our g-band
imaging. Their optical emission is extended, and we tentatively detect a
radio-optical alignment effect in this sample. A subset of our sample (three
sources) shows broad-band spectral energy distributions that flatten out near
the wavelength range of the g-band, implying a dominant contribution there due
to young stars and/or scattered or reprocessed radiation from the active
nucleus.
