Based on observations collected at the European Southern Observatory (Chile) and at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.

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Tables 5-8 are only available in electronic form at http://www.aanda.org

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... stars

The sample also includes three very young planetary nebulae, which are here also considered post-AGB stars in a broad sense.

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... lines

Note that the initial strategy was to use the spectrum of a hot, rapidly rotating star observed on the same night for this purpose, but it was found that the latter showed faint but detectable DBs in its spectrum, which did not allow us to perform this correction properly. In contrast, the 6284, 6993, and 7224 Å DBs were found to be totally absent in HD 172324.

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... stars

Stars with spectral types later than G- were discarded for the analysis, as their continuum is dominated by the molecular bands, which makes identifying DBs a very difficult task.

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... class I

A luminosity class I is adopted because this is the class corresponding to low-gravity stars. Note that post-AGB stars only look like super-giants, but they are not genuine population I massive super-giant stars (Fitzgerald 1970). Values quoted by different authors are generally in good agreement, and the discrepancies found, when relevant, are reflected in the associated errors provided in Table 2.

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... (B-A)

Note that among the subgroup of early-type stars we have also included the few sources in Table 2 that are classified as planetary nebulae.

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