  We have made the first detection of circumstellar absorption lines of the
12C13C (Phillips) system 1-0 band and the 12C16O first-overtone 2-0 band in the
spectrum of the post-AGB star HD56126 (IRAS07134+1005). The rotational
temperatures are lower for molecules with a higher permanent dipole moment.
Derived relative column densities ratios are 12C12C/12C13C=36+-13,
12C16O/(12C12C+12C13C)=606+-230, and 12C16O/(12C14N+13C14N)=475+-175.
  The isotopic exchange reaction for 12C12C is too slow to significantly alter
the 12C12C/12C13C ratio and the 12C12C to 12C13C ratio a good measure of half
the carbon isotope ratio: 12C/13C=2 X 12C12C/12C13C=72+-26.
  A fit of the 12C12C excitation model of van Dishoeck & Black (1982) to the
relative population distribution of 12C12C yields n sigma / I = 3.3 +- 1.0 X
1e-14. At r=1e16cm this translates in n=1.7e7cm-3 and dM/dt=2.5e-4Msol/year.
