We present a list of post-AGB stars showing molecular line absorption and emission in the optical spectrum .
Two objects show CH ^ { + } , one in emission and one in absorption , and ten stars show C _ { 2 } and CN in absorption .
The Doppler velocities of the C _ { 2 } lines and the rotational temperatures indicate that the line forming region is the AGB remnant .
An analysis of the post-AGB stars of which CO millimeter data is available suggests that the C _ { 2 } expansion velocity is of the same order as the CO expansion velocity .
HD 56126 has been studied in detail and we find a mass-loss rate of \dot { M } = 2.8 \times 10 ^ { -4 } M _ { \odot } yr ^ { -1 } , f _ { C _ { 2 } } = 2.4 \times 10 ^ { -8 } and f _ { CN } = 1.3 \times 10 ^ { -8 } .
The mass loss derived from C _ { 2 } is significantly larger than \dot { M } = 1.2 \times 10 ^ { -5 } M _ { \odot } yr ^ { -1 } derived from CO. We find that all objects with the 21 \mu m feature in emission show C _ { 2 } and CN absorption , but not all objects with C _ { 2 } and CN detections show a 21 \mu m feature .

keywords : molecules – physical conditions in AGB remnant – mass-loss history on AGB