The Catalogue of mean UBV data on stars (Mermilliod & Mermilliod 1994) contains 102 stars in the FBS area, with 11.0 < V < 16.5 (bright stars are saturated on the FBS plates and are therefore missed) and U - B < -0.50; 53 are included in the FBS catalogue suggesting that, in this magnitude interval, the completeness of the FBS is 52% (53/102). The completeness of the survey increases from 20% at U - B -0.6 to 80% for U - B < -1.0. The QSO U - B colour changes with z; but most of the changes, at least for z < 2.2, are due to the presence of an emission line in one of the two filters. The U - B colour of the continuum is in the range -0.9 < U - B < -0.7. Slitless spectroscopic surveys are sensitive to the colour of the continuum, unaffected by the emission lines. There are 58 known stars with 14.0 < V < 16.5 and U - B < -0.70 in the FBS area; 39 (67%) have been found by the FBS; we shall adopt this value for the completeness of the FBS.
The PG and FBS samples have about 2250 deg2 in common. Out of the 1103 FBS objects, 618 are within the PG fields, 276 being in the PG sample. Thirty-six PG objects have not been found in the FBS. So 88% (276/312) of the PG objects have been discovered. The 36 undiscovered objects have been examined on the FBS plates; 24 have a weak UV excess (the PG survey finds a significant fraction of stars with U - B -0.4, while the FBS is relatively insensitive for U - B > -0.7). From this, we conclude that the FBS is 95% complete for U - B < -0.5. This is significantly larger than the 67% success rate obtained from the UBV stars; it is probably due to the fact that, in principle, the PG survey contains only objects brighter than B = 16.2. There are 25 PG QSOs in the FBS area, 23 of them have been found, confirming that the FBS is very efficient in discovering bright QSOs.
|X: in the RASS-BSC; x: in the WGACAT; R: in the NVSS; (1): error of the ROSAT or VLA position (in arcsec); (2): distance between the FBS and ROSAT or VLA positions (in arcsec); Y: in the PG area; N: not in the PG area; O: APS O magnitudes.|