The NOT GL survey for multiply imaged quasars

A gravitational lens (GL)-search program, initiated in 1990 at the Nordic Optical Telescope (NOT), has revealed several possible GL-candidates among a sample of 168 quasars (QSOs), chosen from three lists compiled by C. Hazard, D. Reimers and J. Surdej, respectively. Some of these candidates, selected for having close companions (within 5 arcseconds), were imaged in several filters and their colours compared. Low dispersion spectra of the most promising candidates were also obtained at the NOT and ESO New Technology Telescope (NTT). None of these has proved to be strong candidates of gravitational lensing effects. We present this new sample of QSOs and combine it with previously published optical QSO samples in a statistical analysis to yield constraints on flat cosmologies and galaxy velocity dispersions. Finally, by simulating larger samples of quasars and gravitational lenses, we discuss how the uncertainties affecting our present results would be changed.