  We describe a search for redshifted [C II] in a z=3.137 damped Ly-alpha
absorption system that has a large neutral hydrogen column density and which
was controversially reported to be a source of CO emission, indicative of rapid
star-formation (Frayer, Brown & Vanden Bout 1994; Braine, Downes & Guilloteau
1996). There is no sign of [C II] emission in our spectrum, which was obtained
during excellent observing conditions at the James Clerk Maxwell Telescope
(JCMT) and covers 1890 km/s. The upper limit we have placed on the integrated
line intensity (3 sigma(T_MB) < 5.9 K km/s for a profile akin to that of the CO
lines) constrains the [C II]/CO(1-0) line-intensity ratio to 3 sigma < 8300,
based on the line intensity reported by Frayer et al. (1994), or to 3 sigma <
58700 based on the data obtained by Braine et al. (1996). These limits are
consistent with values measured in the Galactic plane and for nearby starburst
nuclei; the former, however, is significantly lower than the ratio found in
low-metallicity systems such as the Large Magellanic Cloud (which might be
expected to have much in common with a damped Ly-alpha absorption system at
high redshift). This can be taken as evidence against the reality of the CO
line detections, with the proviso that a system significantly larger than
present-day disk galaxies would NOT have been fully covered by our small beam
whereas it WOULD have been properly sampled by the Frayer et al. observations.
Finally, we demonstate (as did Ivison et al. 1996) that knitting together
overlapping bands can generate erroneous results - specifically, an emission
feature that has a width, profile and central velocity consistent with the
controversial CO emission lines and which could have drawn us to entirely the
wrong conclusions.
