We present the detection of molecular absorption lines in the optical spectrum of the post-AGB star HD 56126 .
The C _ { 2 } Phillips A ^ { 1 } \Pi _ { u } - X ^ { 1 } \Sigma ^ { + } _ { g } ( 1,0 ) , ( 2,0 ) , and ( 3,0 ) ; Swan d ^ { 3 } \Pi _ { g } - a ^ { 3 } \Pi _ { u } ( 0,0 ) and ( 1,0 ) ; and CN Red system A ^ { 2 } \Pi - X ^ { 2 } \Sigma ^ { + } ( 1,0 ) , ( 2,0 ) , ( 3,0 ) , and ( 4,0 ) bands have been identified .
From the identification of the molecular bands we find an expansion velocity of 8.5 \pm 0.6 km s ^ { -1 } ~ { } independent of excitation condition or molecular specie .
On the basis of the expansion velocity , rotational temperatures , and molecular column densities we argue that the line-forming region is the AGB remnant .
This is in agreement with the expansion velocity derived from the CO lines .
We find column densities of \log N _ { C _ { 2 } } = 15.3 \pm 0.3 cm ^ { -2 } and \log N _ { CN } = 15.5 \pm 0.3 cm ^ { -2 } , and rotational temperatures of T _ { rot } = 242 \pm 20 K and T _ { rot } = 24 \pm 5 K respectively for C _ { 2 } and CN .

By studying molecular line absorption in optical spectra of post-AGB stars we have found a new tracer of the AGB remnant .
From comparison with the results of CO and IR observations it is possible to obtain information on non-spherical behavior of the AGB remnant .
Using different molecules with different excitation conditions it should be possible to study the AGB remnant as a function of the distance to the star , and thus as a function of the evolutionary status of the star on the AGB .