  Using UV spectra obtained with FUSE, HST, and/or IUE, we determine
interstellar column densities of 12CO, 13CO, and/or C_2 for ten Galactic sight
lines with 0.37<E(B-V)<0.72. The N(CO)/N(H_2) ratio varies over a factor of 100
in this sample, due primarily to differences in N(CO). For a given N(H_2),
published models of diffuse and translucent clouds predict less CO than is
observed. The J=1-3 rotational levels of 12CO are sub-thermally populated in
these sight lines, with T_ex typically between 3 and 7 K. In general, there is
no significant difference between the excitation temperatures of 12CO and 13CO.
Fits to the higher resolution CO line profiles suggest that CO (like CN) is
concentrated in relatively cold, dense gas. We obtain C_2 column densities from
the F-X (1-0) and (0-0) bands (1314 and 1341 A), the D-X (0-0) band (2313 A),
and the A-X (3-0) and (2-0) bands (7719 and 8757 A). Comparisons among those
N(C_2) yield a set of mutually consistent f-values for the UV and optical C_2
bands, but also reveal some apparent anomalies within the F-X (0-0) band. Both
the kinetic temperature inferred from the C_2 rotational populations (up to
J=18) and the excitation temperature T_02(C_2) are generally smaller than the
corresponding T_01(H_2). Incorporating additional data for K I, HD, CH, C_2,
C_3, CN, and CO from the literature (for a total sample of 74 sight lines), we
find that (1) CO is most tightly correlated with CN; (2) the ratios 12CO/H_2
and 13CO/H_2 both are fairly tightly correlated with the density indicator
CN/CH (but C_2/H_2 is not); and (3) the ratio 12CO/13CO is somewhat
anti-correlated with both CN/CH and N(CO). Sight lines with 12CO/13CO below the
average local Galactic value of 12C/13C appear to sample colder, denser gas in
which isotope exchange reactions have enhanced 13CO, relative to 12CO.
