2D distributions of colors (u − g, g − r, r − i, i − z and g − i) for objects classified as PLO in SDSS photometric catalog (blue lines for contours) and for objects spectroscopically classified as QSO (red solid lines for contours). The PSF magnitudes (ugriz) have been corrected for Galactic extinction according to the model of Schlegel et al. (1998).
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