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1 Introduction
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2 X-ray observations
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3 X-ray analysis
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4 Optical observations
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5 Cross-correlations
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6 X-ray/optical analysis
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7 Sources with no candidate optical counterpart
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8 Summary and conclusions
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References
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Online Material
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Appendix A: Correction of the Classification Scheme
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Appendix B: Measurement of the X-ray-to-optical flux ratios
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