We have carried out extensive radio and optical follow-up of 176 sources from
the 15 GHz 9th Cambridge survey. Optical identifications have been found for
155 of the radio sources; optical images are given with radio maps overlaid.
The continuum radio spectrum of each source spanning the frequency range 1.4 -
43 GHz is also given. Two flux-limited samples are defined, one containing 124
sources complete to 25 mJy and one of 70 sources complete to 60 mJy. Between
one fifth and one quarter of sources from these flux-limited samples display
convex radio spectra, rising between 1.4 and 4.8 GHz. These rising-spectrum
sources make up a much larger fraction of the radio source population at this
high selection frequency than in lower frequency surveys. We find that by using
non-simultaneous survey flux density measurements at 1.4 and 15 GHz to remove
steep spectrum objects, the efficiency of selecting objects with spectra rising
between 1.4 and 4.8 GHz (as seen in simultaneous measurements) can be raised to
49 percent without compromising the completeness of the rising spectrum sample.Comment: 40 pages, 223 figures. Accepted for publication in MNRAS. Version
with full size images (A4 paper) avaliable at
http://www.mrao.cam.ac.uk/publications/papers files ME777.ps and ME777.pd