  We investigate the nature of the faintest radio sources detected in 3 VLA
surveys, to faint limits 8 microJy at 8.5 GHz. Using the Keck Low Resolution
Imaging Spectrograph in BRI and the Near Infra Red Camera in K, we image 51
radio sources, and identify probable optical counterparts for 50. With LRIS
spectroscopy, we successfully acquire new redshifts for 17 sources, and 9 have
redshifts from previous data. Of this sample of 26, about 60 per cent are disk
galaxies with high radio to optical ratios, indicating ongoing or very recent
starbursts, at redshifts z=0-1. Others are normal, non-interacting galaxies at
z<0.4, QSOs and radioluminous giant ellipticals (obscured AGN?). All of the 14
non-QSO galaxies with spectra show emission lines, but with luminosities
corresponding to star-formation rates about an order of magnitude lower than
estimated from the high radio luminosities (dust extinction?). Three have
strong H-delta absorption (`e+a galaxies'). The Keck spectra also provide
approximate emission line widths and rotation curves. Most (12/14) have
kinematics implying large dynamical masses, and moderate excitation ratios
consistent with L* spirals. The other two, which have sub-L* optical
luminosities, appear to be low-mass galaxies with much higher excitations and
may be more similar to HII galaxies.
