A QSO survey via optical variability and zero proper motion in the M92 field - IV. More QSOs due to improved photometry

We continue the QSO search in the 10 square degrees Schmidt field around M92 based on variability and proper motion (VPM) constraints. We have re-reduced 162 digitised B plates with a time-baseline of more than three decades and have considerably improved both the photometric accuracy and the star-galaxy separation at B > 19. QSO candidates are selected and marked with one out of three degrees of priority based on the statistical significance of their measured variability and zero proper motion. Spectroscopic follow-up observations of 84 new candidates with B > 19 revealed an additional 37 QSOs and 7 Seyfert 1s. In particular, all 92 high-priority candidates are spectroscopically classified now; among them are 70 QSOs and 9 Seyfert 1s (success rate 86%). We expect that 87% (55%) of all QSOs with B < 19.0 (19.8) are contained in this high-priority subsample. For the combined sample of high-priority and medium-priority objects, a completeness of 89% is estimated up to B-lim = 19.5. The sample of all AGNs detected in the framework of the VPM search in the M92 field contains now 95 QSOs and 14 Seyfert 1s with B <= 19.9. Although the VPM QSOs were selected by completely different criteria, their properties do not significantly differ from those of QSOs found by more traditional optical survey techniques. In particular, the spectra and the optical broad band colours do not provide any hints on a substantial population of red QSOs up to the present survey limit.