2 The observations

The polarimetric observations were carried out during several runs at the European Southern Observatory (ESO La Silla, Chile) from 1996 to 1999, using the 3.6 m telescope equipped with the EFOSC camera and spectrograph. In 1996, the instrument was EFOSC1 and the detector a 512$\times$512 TeK CCD (ESO#26) with a pixel size of 27 $\mu$m corresponding to 0 $\hbox{$.\!\!^{\prime\prime}$ }$605 on the sky (Melnick et al. 1989). Later, EFOSC1 was replaced by EFOSC2, and the detector was a 2048$\times$2048 Loral/Lesser CCD (ESO#40) with a pixel size of 15 $\mu$m corresponding to 0 $\hbox{$.\!\!^{\prime\prime}$ }$157 on the sky (Patat 1999). The latter CCD was used in a 2$\times$2 binning mode, except in October 98.

With EFOSC, polarimetry is performed by inserting in the parallel beam a Wollaston prism which splits the incoming light rays into two orthogonally polarized beams. Each object in the field has therefore two images on the CCD detector, separated by 10 $\hbox{$^{\prime\prime}$ }$ or 20 $\hbox{$^{\prime\prime}$ }$(depending on the Wollaston), and orthogonally polarized. To avoid image overlapping, one puts at the telescope focal plane a special mask made of alternating transparent and opaque parallel strips whose width corresponds to the splitting. The object is positioned at the centre of a transparent strip which is imaged on a region of the CCD chosen as clean as possible. The final CCD image then consists of alternate orthogonally polarized strips of the sky, two of them containing the polarized images of the object itself (di Serego Alighieri 1989, 1998; Lamy & Hutsemékers 1999). Note that the polarization measurements do not depend on variable transparency or seeing since the two orthogonally polarized images of the object are simultaneously recorded. The 20 $\hbox{$^{\prime\prime}$ }$ Wollaston was used during the September 96, April 98, and September 99 observing runs, while the 10 $\hbox{$^{\prime\prime}$ }$ Wollaston was used in October 98 and April 99.

In order to derive linear polarization measurements, i.e. the two normalized Stokes parameters q and u, frames must be obtained with at least two different orientations of the Wollaston. With EFOSC1, this was done by rotating the whole instrument by 45 $\hbox{$^\circ$ }$ (usually at the adapter angles 270 $\hbox{$^\circ$ }$ and 225 $\hbox{$^\circ$ }$) such that, for each object, two frames were secured (Melnick et al. 1989). The excellent transmission of the Wollaston makes these two orientations sufficient (di Serego Alighieri 1989). With EFOSC2, a half-wave plate (HWP) is inserted in the optical path and four frames with the HWP at position angles 0 $\hbox{$^\circ$ }$, 22.5 $\hbox{$^\circ$ }$, 45 $\hbox{$^\circ$ }$, and 67.5 $\hbox{$^\circ$ }$ were obtained, without the need of rotating the whole instrument (Schwarz & Guisard 1995).

Most observations were done with the Bessel V filter. A few additional ones were obtained with the Gunn i filter. Typical exposure times are around 5 minutes per frame. Seeing was around 1 $\hbox{$.\!\!^{\prime\prime}$ }$2 except in September 99 ( $\sim 5\hbox{$^{\prime\prime}$ }$). In addition, polarimetric calibration stars were observed in the same filters in order to unambiguously fix the zero-point of the polarization position angle and to check the whole observing and reduction process. The observed standard stars are given in Table 1.