Recent optical spectra of post-AGB stars show the presence of C _ { 2 } , CN , and CH ^ { + } originating in the circumstellar shell .
We present here new , higher resolution spectra which provide constraints on the physical parameters and information on the line profiles .
An empirical curve of growth for the C _ { 2 } Phillips and CN Red system lines in the spectrum of HD 56126 yields b = 0.50 ^ { +0.59 } _ { -0.23 } km s ^ { -1 } .
CH ^ { + } ( 0,0 ) emission lines in the spectrum of the Red Rectangle have been resolved with a FWHM \approx 8.5 \pm 0.8 km s ^ { -1 } .
The circumstellar CN lines of IRAS 08005–2356 are resolved into two separate components with a velocity separation of \Delta v = 5.7 \pm 2.0 km s ^ { -1 } .
The line profiles of CN of HD 235858 have not been resolved .