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$\begin{figure} \par\includegraphics[width=8.4cm,clip]{5282fig1.eps}\end{figure}$ Figure 1: Removal of telluric lines at 6284, 6993, and 7224 Å. A few examples are shown before ( left) and after correction ( right). In the middle panel we show the stellar spectrum of HD 172324 (B9Ib), one of the sample stars, which was used as telluric divisor (see text).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{5282fig2.eps}\end{figure}$ Figure 2: Equivalent width measurements taken from the literature for field stars, plotted as a function of E(B-V). Table 3 gives slopes EW/E(B-V) and correlations r for the linear fits.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{5282fig3.eps}\end{figure}$ Figure 3: Sample spectra showing the region around the DB at 6284 Å, before ( left panel) and after ( right panel) removal of the telluric lines using HD 172324 as the telluric divisor. Dotted lines indicate the continuum level adopted in each case.
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{5282fig4.eps}\end{figure}$ Figure 4: Equivalent width (in Å) of the 9 DBs selected for analysis as a function of E(B-V) for the post-AGB stars in the sample. Solid lines correspond to the fits derived in this paper for field stars dominated by interstellar extinction (see also Sect. 4.1). The inverted triangles represent upper limits.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{5282fig5.eps}\end{figure}$ Figure 5: E(B-V) versus Galactic latitude distribution of the post-AGB stars in the sample (circles) and for reference stars taken from Guarinos (1988a,1997,1988b) (small squares). Post-AGB stars dominated by circumstellar extinction (DCS-type) are indicated by filled circles and arelabelled with their IRAS name.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{5282fig6.eps}\end{figure}$ Figure 6: Equivalent width of the 9 DBs selected for analysis as a function of E(B-V) for the post-AGB stars in the sample in which filled circles represent the subsample of post-AGB stars dominated by circumstellar extinction (DCS-type) and open triangles the rest of stars in the sample. Solid lines again correspond to the fits derived in this paper for field stars dominated by interstellar extinction. The arrows indicate upper limits.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fig7.eps}\end{figure}$ Figure 7: Equivalent width of the 6284 Å band vs. E(B-V) for the group of stars dominated by circumstellar extinction ( left panel; DCS type) and for the rest of stars in the sample ( right panel) with different symbols indicating the dominant chemistry and spectral type of the observed stars. The solid line represents the behaviour observed in field stars dominated by interstellar extinction, as deduced from the data shown in Fig. 2.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fig8.eps}\end{figure}$ Figure 8: Same as Fig. 7, for the 5780 Å band.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fig9.eps}\end{figure}$ Figure 9: Same as Fig. 7, for the 7224 Å band.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg10.eps}\end{figure}$ Figure 10: Same as Fig. 7, for the 6614 Å band.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg11.eps}\end{figure}$ Figure 11: Same as Fig. 7, for the 5797 Å band.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg12.eps}\end{figure}$ Figure 12: Same as Fig. 7, for the 6993 Å band.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg13.eps}\end{figure}$ Figure 13: Same as Fig. 7, for the 6379 Å band.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg14.eps}\end{figure}$ Figure 14: Same as Fig. 7, for the 5850 Å band.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg15.eps}\end{figure}$ Figure 15: Same as Fig. 7, for the 6196 Å band.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{5282fg16.eps}\end{figure}$ Figure 16: The complex profile of the Na I D₁ line, as observed in IRAS 04296+3429.
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In the text

$\begin{figure} \par\includegraphics[width=6.5cm,clip]{5282f17a.eps}\end{figure}$ Figure 17: Velocities of the DBs and the stellar and interstellar absorption lines and the nebular emission line components plotted per panel for each target. The y-axis is in arbitrary units. The x-axis is the LSR velocity in km s^-1, with big ticks separated by 50 km s^-1. Note that the widths of the panels are identical, i.e. 200 km s^-1, but the central velocity of each is shifted to show all lines for each target. Error bars are $\leq$ 10 km s^-1 for stellar lines, $\leq$ 20 km s^-1 for DBs, and $\leq$ 5 km s^-1 for the sodium and potassium components. Targets dominated by circumstellar reddening are labelled ``DCS'' and high radial velocity targets are labelled ``HV'' The stellar and DB velocity components can be compared directly to those of neutral sodium and potassium in either emission ( upward arrow) or absorption ( downward arrow) plotted at the top of each panel.
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In the text

$\begin{figure} \par\includegraphics[width=6.5cm,clip]{5282f17b.eps}\\ \vspace*{6mm} \includegraphics[width=6.5cm,clip]{5282f17c.eps}\end{figure}$ Figure 17: continued.
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In the text

$\begin{figure} \par\includegraphics[width=6.5cm,clip]{5282fg18.eps}\end{figure}$ Figure 18: The 5850 $\AA$ band observed in the high radial velocity ( $\sim$ 100 km s^-1) sample star IRAS 19114+0002 (solid line). The non-detection of this feature at the Doppler-shifted velocity corresponding to the central star (dashed line) confirms the interstellar nature of the band and the absence of a circumstellar component. The potassium and sodium velocity structures are shown in the bottom panels to illustrate the distribution of interstellar medium in this line of sight. A narrow sodium emission line is seen in the middle panel. The two arrows in the bottom most panel indicate two telluric features next to the potassium line.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{5282fig1.eps}\end{figure}$	Figure 1: Removal of telluric lines at 6284, 6993, and 7224 Å. A few examples are shown before ( left) and after correction ( right). In the middle panel we show the stellar spectrum of HD 172324 (B9Ib), one of the sample stars, which was used as telluric divisor (see text).
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{5282fig2.eps}\end{figure}$	Figure 2: Equivalent width measurements taken from the literature for field stars, plotted as a function of E(B-V). Table 3 gives slopes EW/E(B-V) and correlations r for the linear fits.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{5282fig3.eps}\end{figure}$	Figure 3: Sample spectra showing the region around the DB at 6284 Å, before ( left panel) and after ( right panel) removal of the telluric lines using HD 172324 as the telluric divisor. Dotted lines indicate the continuum level adopted in each case.
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$\begin{figure} \par\includegraphics[width=12cm,clip]{5282fig4.eps}\end{figure}$	Figure 4: Equivalent width (in Å) of the 9 DBs selected for analysis as a function of E(B-V) for the post-AGB stars in the sample. Solid lines correspond to the fits derived in this paper for field stars dominated by interstellar extinction (see also Sect. 4.1). The inverted triangles represent upper limits.
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$\begin{figure} \par\includegraphics[width=8.5cm,clip]{5282fig5.eps}\end{figure}$	Figure 5: E(B-V) versus Galactic latitude distribution of the post-AGB stars in the sample (circles) and for reference stars taken from Guarinos (1988a,1997,1988b) (small squares). Post-AGB stars dominated by circumstellar extinction (DCS-type) are indicated by filled circles and arelabelled with their IRAS name.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fig8.eps}\end{figure}$	Figure 8: Same as Fig. 7, for the 5780 Å band.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fig9.eps}\end{figure}$	Figure 9: Same as Fig. 7, for the 7224 Å band.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg10.eps}\end{figure}$	Figure 10: Same as Fig. 7, for the 6614 Å band.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg11.eps}\end{figure}$	Figure 11: Same as Fig. 7, for the 5797 Å band.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg12.eps}\end{figure}$	Figure 12: Same as Fig. 7, for the 6993 Å band.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg13.eps}\end{figure}$	Figure 13: Same as Fig. 7, for the 6379 Å band.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg14.eps}\end{figure}$	Figure 14: Same as Fig. 7, for the 5850 Å band.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{5282fg15.eps}\end{figure}$	Figure 15: Same as Fig. 7, for the 6196 Å band.
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{5282fg16.eps}\end{figure}$	Figure 16: The complex profile of the Na I D₁ line, as observed in IRAS 04296+3429.
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$\begin{figure} \par\includegraphics[width=6.5cm,clip]{5282f17b.eps}\\ \vspace*{6mm} \includegraphics[width=6.5cm,clip]{5282f17c.eps}\end{figure}$	Figure 17: continued.
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