and R. Lucas 
. NRAO,
520 Edgemont Road,
Charlottesville, VA,
USA 22903-2475 IRAM,
300 Rue de la Piscine,
F-38406 Saint Martin d'Hères,
France 
2.6mm and 1.3mm CO absorption
and emission along nine lines of sight toward compact extragalactic mm-wave
continuum sources, using the IRAM Plateau de Bure Interferometer and NRAO 12m
telescopes. In absorption we detected some two dozen kinematic components,
and nearly every feature known from HCO 
spectra
has a direct CO counterpart: a few CO lines are missing or
very weak in emission even when 
.
The column densities of CO and HCO are well correlated, but not linearly
related. The widths of the CO lines are typically 15% smaller than those
of HCO (on average 0.75 kms 
vs. 0.86 kms ).
We derive 
CO column densities
which are in all cases
very small compared to the column of carbon nuclei expected for 1 magnitude
of visual extinction, even allowing for substantial depletion. The partial
thermal pressure of 
is inferred to be
K,
with a median p/k = 
K.
Thus the clouds are likely warm ( 
tens of K), somewhat diffuse
( 
), with the majority of the gas-phase carbon in
the form of C and perhaps even with a substantial fraction of H I in
the thinnest cases.
The isotope ratios in the CO usually differ strongly from the local
interstellar ratio which we have separately measured in these clouds
to be C/ 
C = 60
; we find
N 
N 
,
declining with increasing N( CO). The
CO/C 
O ratio seen in emission or absorption is typically 25 (instead
of 8) and C O is very difficult to detect in emission even when
. Apparently, the relative abundance of CO is
typically greatly enhanced, even at very low extinction, and never diminished by
selective photodissociation. One effect of this enhancement is
that lines of CO are substantially less optically thick than might
otherwise have been inferred. There is little evidence for a general
selective depletion of C O.
Preprint requests: lucas@iram.fr