  Aims: Our aims are threefold: a) To compare the $uv$ and mm-wave results; b)
to interpret 13CO and 12CO abundances in terms of the physical processes which
separately and jointly determine them; c) to interpret observed J=1-0
rotational excitation and line brightness in terms of ambient gas properties.
  Methods: A simple phenomenological model of CO formation as the immediate
descendant of quiescently-recombining HCO+ is used to study the accumulation,
fractionation and rotational excitation of CO in more explicit and detailed
models of H2-bearing diffuse/H I clouds
  Results: The variation of N(CO) with N(H2) is explained by quiescent
recombination of a steady fraction n(HCO+)/n(H2) = 2 x 10^{-9}. Observed
N(12CO))/N(13CO) ratios generally do not require a special chemistry but result
from competing processes and do not provide much insight into the local gas
properties, especially the temperature. J=1-0 CO line brightnesses directly
represent N(CO), not N(H2), so the CO-H2 conversion factor varies widely; it
attains typical values at N(12CO) \la 10^{16}cm^{-2}. Models of CO rotational
excitation account for the line brightnesses and CO-H2 conversion factors but
readily reproduce the observed excitation temperatures and optical depths of
the rotational transitions only if excitation by H-atoms is weak -- as seems to
be the case for the very most recent calculations of these excitation rates.
