  We present the detection of molecular absorption lines in the optical
spectrum of the post-AGB star HD56126. The C2 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+-0.6 kms 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_C2=15.3+-0.3 cm-2 and log
N_CN=15.5+-0.3 cm-2, and rotational temperatures of T_rot=242+-20 K and
T_rot=24+-5 K respectively for C2 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.
