1 Introduction

Considering a sample of 170 optically polarized quasars with accurate polarization measurements, we recently found that quasar polarization vectors are not randomly oriented on the sky as naturally expected. Indeed, in some regions of the three-dimensional Universe (i.e. in regions delimited in right ascension, declination, and redshift), the quasar polarization position angles appear concentrated around preferential directions, suggesting the existence of large-scale coherent orientations -or alignments- of quasar polarization vectors (Hutsemékers 1998, hereafter Paper I). Mainly because the polarization vectors of objects located along the same line of sight but at different redshifts are not accordingly aligned, possible instrumental bias and contamination by interstellar polarization are unlikely to be responsible for the observed effect (cf. Paper I for a more detailed discussion). The very large scale at which these coherent orientations are seen suggests the presence of correlations in objects or fields on spatial scales $\sim$1000h^-1 Mpc at redshifts $z \simeq 1{-}2$, possibly unveiling a new effect of cosmological importance. Although we found this orientation effect statistically significant, the sample is not very large, and further investigation is needed to confirm it, especially in view of its very unexpected nature. One of the simplest tests consists in measuring the polarization of a new sample of quasars located in one of the regions of the sky where an alignment was previously found and where we can predict in advance the range of polarization position angles. Such polarimetric observations have been recently carried out, and the results are presented here. In Sect. 2, we present the new polarimetric observations, as well as a compilation of the most recent measurements from the literature. The results of the statistical analysis -confirming the orientation effect- are given in Sect. 3. Then, from the properties of the objects in the region of alignment we derive some constraints on possible interpretations. These are discussed in Sect. 4. Conclusions form the last section.