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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 10 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 M=2.8 x 10(-4) M.yr(-1), f(C2) = 2.4 x 10(-8) and f(CN) = 1.3 x 10(-8). The mass-loss derived from C-2 is significantly; larger than M = 1.2 x 10(-5) M.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.

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 10 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 M=2.8 x 10(-4) M.yr(-1), f(C2) = 2.4 x 10(-8) and f(CN) = 1.3 x 10(-8). The mass-loss derived from C-2 is significantly; larger than M = 1.2 x 10(-5) M.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.