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$\begin{figure} \par\includegraphics[width=8.4cm,clip]{0607fig1.ps} \end{figure}$ Figure 1: Observed R-parameter values as a function of the cluster [Fe/H] for the two adopted metallicity scales. The theoretical calibration for Y=0.245and a cluster age of 13 Gyr is also shown.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{0607fig2.ps} \end{figure}$ Figure 2: He abundance values as a function of [Fe/H] for the 57 clusters analyzed. The two panels refer to the two metallicity scales adopted in our analysis. The He abundances determined for the clusters with [Fe/H] in the range comprised between the two vertical dotted lines are affected by the choice of the cluster age. The horizontal dashed line denotes the most probable value for $Y{_{\rm GGC}}$ (see text for details). Filled circles denote clusters with ${\it HB}_{\rm type}\geq 0.8$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0607fig3.ps} \end{figure}$ Figure 3: Probability Distribution Function (PDF) for the He abundance in our GGC sample when using the ZW84 metallicity scale. The dotted line represents the PDF for the 51 clusters with [Fe/H] in the range not affected by the cluster age (see text); the solid line represents the subsample with ${\it HB}_{\rm type}<0.8$ , while the dashed line represents the subsample with ${\it HB}_{\rm type}\geq 0.8$ . The individual PDFs are not normalized, but for each value of Y the sum of the PDFs for the two subsamples selected on the basis of their HB colour is equal to the PDF for the whole sample.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{0607fig4.ps} \end{figure}$ Figure 4: As in Fig. 3, but for the CG97 [Fe/H] scale (see text).
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{0607fig1.ps} \end{figure}$	Figure 1: Observed R-parameter values as a function of the cluster [Fe/H] for the two adopted metallicity scales. The theoretical calibration for Y=0.245and a cluster age of 13 Gyr is also shown.
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$\begin{figure} \par\includegraphics[width=8.4cm,clip]{0607fig4.ps} \end{figure}$	Figure 4: As in Fig. 3, but for the CG97 [Fe/H] scale (see text).
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