All Figures

$\begin{figure} \par\includegraphics[clip,width=7.9cm]{HISTO_FX.eps}\end{figure}$ Figure 1: Histogram of the logarithm of the X-ray count rates (in counts s^-1) of the 29 321 ROSAT-FSC sources.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.2cm]{CARTE.eps}\end{figure}$ Figure 2: Distribution over the sky of a) the 29 321 ROSAT-FSC sources, b) the 15 848 sources brighter than 0.02 counts s^-1.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8cm]{ROSAT_PPM.eps}\end{figure}$ Figure 3: Position differences for the coincidences between ROSAT-FSC X-ray sources and PPM stars within two arcminutes. The radius of the circle is 25 $\hbox {$^{\prime \prime }$ }$ .
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.2cm]{histo_dis.eps}\end{figure}$ Figure 4: Histogram of the square of the separation between the X-ray and USNO positions for the 25 549 associations in units of 100 square arcsec.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.6cm]{MAG_MAG.eps}\end{figure}$ Figure 5: Comparison of the USNO and APS O magnitudes for starlike objects ( left panel) and for galaxies ( right panel). The solid lines represent the best linear fits between the two sets of data; the dotted lines have a slope of unity.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.5cm]{COMP_MAG.eps}\end{figure}$ Figure 6: Comparison of the USNO and APS O magnitudes with the NPM1 O magnitudes for 95 galaxies (filled squares) and 81 stars (open circles). The dotted lines have a slope of unity.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.4cm]{SP_OHP.eps}\end{figure}$ Figure 7: Spectra of the six Seyfert 1, the three Seyfert 2 and three of the five starburst galaxies confirmed by our spectroscopic observations at BAO and OHP.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.4cm]{COLMAG_BIG.eps}\end{figure}$ Figure 8: a) APS O-E colour indices vs. O magnitudes for all starlike candidate identifications within 35 $\hbox {$^{\prime \prime }$ }$ ; b) the same for coincidences within 15 $\hbox {$^{\prime \prime }$ }$ ; c) the same for the A to G stars (filled circles) and the K and M stars (open triangles) as classified according to the Hamburg scheme; d) the same for the BLUE-WK and EBL-WK objects (filled triangles), the RED-WK objects (open triangles) and the QSOs (filled circles).
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In the text

$\begin{figure} \par\includegraphics[clip,width=4.8cm]{colmag_qso.eps}\end{figure}$ Figure 9: APS O-E colour indices vs. O magnitudes for 68 confirmed QSOs.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.5cm]{POSCOL.eps}\end{figure}$ Figure 10: Position differences between the ROSAT-FSC X-ray sources and APS starlike objects located in each of the four zones previously defined. The circles have a radius of 15 $\hbox {$^{\prime \prime }$ }$ and 35 $\hbox {$^{\prime \prime }$ }$ , respectively.
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In the text

$\begin{figure} \par\includegraphics[clip,width=6.9cm]{SP_RAUL_CV.eps}\end{figure}$ Figure 11: Spectrum of a cataclysmic variable observed at OAGH.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.4cm]{SP_RAUL_1.eps}\end{figure}$ Figure 12: Spectra of 18 QSOs from Table 4 observed at OAGH.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.4cm]{SP_RAUL_2.eps}\end{figure}$ Figure 13: Spectra of 18 QSOs from Table 4 observed at OAGH.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.4cm]{SP_RAUL_3.eps}\par\end{figure}$ Figure 14: Spectra of six objects from Table 4 (one Seyfert 1, one QSO, two BL Lac objects and two NLS1s) observed at OAGH.
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In the text

$\begin{figure} \par\includegraphics[clip,width=8.4cm]{FXFOPT.eps}\end{figure}$ Figure 15: Plot of log( $F_{\rm X}/F_{V}$ ) vs. $O_{\rm USNO}$ for 552 radio quiet QSOs. The two diagonal lines correspond to the sensitivity limits of the BSC and FSC ROSAT survey (0.05 and 0.02 count s^-1, respectively).
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In the text

$\begin{figure} \par\includegraphics[clip,width=7.9cm]{HISTO_FX.eps}\end{figure}$	Figure 1: Histogram of the logarithm of the X-ray count rates (in counts s^-1) of the 29 321 ROSAT-FSC sources.
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$\begin{figure} \par\includegraphics[clip,width=8.2cm]{CARTE.eps}\end{figure}$	Figure 2: Distribution over the sky of a) the 29 321 ROSAT-FSC sources, b) the 15 848 sources brighter than 0.02 counts s^-1.
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$\begin{figure} \par\includegraphics[clip,width=8cm]{ROSAT_PPM.eps}\end{figure}$	Figure 3: Position differences for the coincidences between ROSAT-FSC X-ray sources and PPM stars within two arcminutes. The radius of the circle is 25 $\hbox {$^{\prime \prime }$ }$ .
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$\begin{figure} \par\includegraphics[clip,width=8.2cm]{histo_dis.eps}\end{figure}$	Figure 4: Histogram of the square of the separation between the X-ray and USNO positions for the 25 549 associations in units of 100 square arcsec.
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$\begin{figure} \par\includegraphics[clip,width=8.6cm]{MAG_MAG.eps}\end{figure}$	Figure 5: Comparison of the USNO and APS O magnitudes for starlike objects ( left panel) and for galaxies ( right panel). The solid lines represent the best linear fits between the two sets of data; the dotted lines have a slope of unity.
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$\begin{figure} \par\includegraphics[clip,width=8.5cm]{COMP_MAG.eps}\end{figure}$	Figure 6: Comparison of the USNO and APS O magnitudes with the NPM1 O magnitudes for 95 galaxies (filled squares) and 81 stars (open circles). The dotted lines have a slope of unity.
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$\begin{figure} \par\includegraphics[clip,width=8.4cm]{SP_OHP.eps}\end{figure}$	Figure 7: Spectra of the six Seyfert 1, the three Seyfert 2 and three of the five starburst galaxies confirmed by our spectroscopic observations at BAO and OHP.
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$\begin{figure} \par\includegraphics[clip,width=4.8cm]{colmag_qso.eps}\end{figure}$	Figure 9: APS O-E colour indices vs. O magnitudes for 68 confirmed QSOs.
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$\begin{figure} \par\includegraphics[clip,width=8.5cm]{POSCOL.eps}\end{figure}$	Figure 10: Position differences between the ROSAT-FSC X-ray sources and APS starlike objects located in each of the four zones previously defined. The circles have a radius of 15 $\hbox {$^{\prime \prime }$ }$ and 35 $\hbox {$^{\prime \prime }$ }$ , respectively.
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$\begin{figure} \par\includegraphics[clip,width=6.9cm]{SP_RAUL_CV.eps}\end{figure}$	Figure 11: Spectrum of a cataclysmic variable observed at OAGH.
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$\begin{figure} \par\includegraphics[clip,width=8.4cm]{SP_RAUL_1.eps}\end{figure}$	Figure 12: Spectra of 18 QSOs from Table 4 observed at OAGH.
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$\begin{figure} \par\includegraphics[clip,width=8.4cm]{SP_RAUL_2.eps}\end{figure}$	Figure 13: Spectra of 18 QSOs from Table 4 observed at OAGH.
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$\begin{figure} \par\includegraphics[clip,width=8.4cm]{SP_RAUL_3.eps}\par\end{figure}$	Figure 14: Spectra of six objects from Table 4 (one Seyfert 1, one QSO, two BL Lac objects and two NLS1s) observed at OAGH.
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$\begin{figure} \par\includegraphics[clip,width=8.4cm]{FXFOPT.eps}\end{figure}$	Figure 15: Plot of log( $F_{\rm X}/F_{V}$ ) vs. $O_{\rm USNO}$ for 552 radio quiet QSOs. The two diagonal lines correspond to the sensitivity limits of the BSC and FSC ROSAT survey (0.05 and 0.02 count s^-1, respectively).
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