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1 Introduction
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2 The sample
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3 Comparison of the optically bright and optically faint X-ray source samples
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4 Photometric redshift technique applied to an X-ray selected sample
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5 The properties of OFS with spectroscopic redshifts
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6 X-ray-to-optical flux ratios
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7 X-ray spectroscopy
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8 X-ray absorbed QSOs
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9 Ultra deep field
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10 Conclusions
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References
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