All Figures

$\begin{figure} \par\includegraphics[width=7.85cm,clip]{7359fig1.ps} \end{figure}$ Figure 1: The flux distribution for the sources detected in the different bands: T band (no shading), S band (vertical shading), H band (diagonal shading), HH band (horizontal shading).
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{7359fig2.ps} \end{figure}$ Figure 2: $f_{\rm x}(2{-}10)$ keV (hard band H) versus $f_{\rm x}(0.5{-}2)$ keV (soft band S). Solid symbols: objects of known redshift; open symbols: objects without measured redshift; crosses: optically unidentified sources; asterisks: optically identified but unclassified sources; triangles: galaxies; circles: point-like AGNs; squares: variability selected AGN candidates with extended images (BTK); smaller symbols: sources with marginal (M) or ambiguous (A) identifications (see Table 2). Continuous lines represent loci of constant hardness ratio HR. The inset shows the distribution of hardness ratios.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{7359fig3.ps} \end{figure}$ Figure 3: Optical F band flux f_{_F} (also shown as apparent magnitudes F) versus X-ray (2-10 keV) flux $f_{\rm x}$ (2-10 keV). Solid symbols: objects of known redshift; open symbols: objects without measured redshift; crosses: optically unidentified sources; asterisks: optically identified but unclassified sources; triangles: galaxies; circles: point-like AGNs; squares: variability selected AGN candidates with extended images (BTK); smaller symbols: sources with marginal (M) or ambiguous (A) identifications (see Table 2). Arrows: 1- $\sigma$ upper limits. Connected symbols represent ambiguous identifications. Diamond with error bars represents the average fluxes of the X-ray undetected BTK objects, obtained by X-ray image stacking.
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{7359fig4.ps} \end{figure}$ Figure 4: Optical F band luminosity $L_{\rm o}$ versus the X-ray (2-10 keV) band luminosity $L_{\rm x}(2{-}10$ keV). Luminosities are also shown as absolute magnitudes M_F; note that for the objects in figure $\langle B_J-F\rangle \simeq 0.7$ . Solid symbols: objects of know redshift; triangles: galaxies; circles: point-like AGNs; squares: variability selected AGN candidates with extended images (BTK); smaller symbols: sources with marginal (M) or ambiguous (A) identifications (see Table 2). Arrows represent 3- $\sigma$ (instead of 1- $\sigma$ ) upper limits: these occur for both X-ray undetected objects from NED and from BTK (which are marked in this figure with big circles) and for objects detected in X-ray bands other than H (without big circles). The continuous lines represent the indicated constant values of the $\rm X/O$ ratio.
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In the text

$\begin{figure} \par\includegraphics[width=7.85cm,clip]{7359fig5.ps} \end{figure}$ Figure 5: $\rm X/O$ versus hardness ratio HR. Solid symbols: objects of know redshift; open symbols: objects without measured redshift; crosses: optically unidentified sources; asterisks: optically identified but unclassified sources; triangles: galaxies; circles: point-like AGNs; squares: variability selected AGN candidates with extended images (BTK); smaller symbols: sources with marginal (M) or ambiguous (A) identifications (see Table 2), connected in the case of ambiguous identifications. Horizontal and downward vertical arrows: 3- $\sigma$ upper limits derived from X-ray bands; upward vertical arrows: optical limit F>22.
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In the text

$\begin{figure} \par\includegraphics[width=8.1cm,clip]{7359fig6.ps} \end{figure}$ Figure 6: Logarithm of the optical variability, as measured by the the rms magnitude changes $\sigma ^\ast$ , versus the logarithm of the $\rm X/O$ ratio. Symbols as in the previous figures. Arrows: 3- $\sigma$ upper limits.The regression lines shown are computed excluding the upper limits.
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In the text

$\begin{figure} \par\includegraphics[width=7.85cm,clip]{7359fig1.ps} \end{figure}$	Figure 1: The flux distribution for the sources detected in the different bands: T band (no shading), S band (vertical shading), H band (diagonal shading), HH band (horizontal shading).
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$\begin{figure} \par\includegraphics[width=8.1cm,clip]{7359fig6.ps} \end{figure}$	Figure 6: Logarithm of the optical variability, as measured by the the rms magnitude changes $\sigma ^\ast$ , versus the logarithm of the $\rm X/O$ ratio. Symbols as in the previous figures. Arrows: 3- $\sigma$ upper limits.The regression lines shown are computed excluding the upper limits.
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