The B2 sample of radio sources represents an ideal sample for studying the
characteristics of low-luminosity radio galaxies (LLRG). This sample, which
contains about 100 objects, was obtained from identifying of B2 radio
sources with bright elliptical galaxies (see Colla et al. 1975;
Fanti et al.\
1978). Because of the selection criteria, the sample is dominated by
radio galaxies with a power typically between 1022 and 
W Hz-1\
at 20 cm. In the last few years this sample has been extensively used to
investigate the properties of LLRG and to compare them with those of
powerful radio galaxies (PRG).
Some of the objects in the sample are well known and well studied at different
radio frequencies and resolutions (see list in Fanti et al.\
1987). However, most of the sample has been studied in detail only at
20 cm with different VLA configurations (Fanti et al. 1987 and
references therein). The data collected from these studies have allowed a
detailed investigation of the radio characteristics of LLRG with particular
interest in radio jets. The results on the general properties of LLRG,
summarized in de Ruiter et al.\
(1990), show that they are mainly edge-darkened, Fanaroff-Riley
type-1 radio sources. Jets are very often observed in these objects
(
) and they can also be very prominent (unlike in PRG). The jets
also tend to be more symmetrical than in PRG. The characteristics of these
radio jets (opening angle and brightness distribution) are consistent with
their being low-Mach number jets when they are compared with a theoretical
model (Bicknell et al.\
1990). However, a more recent analysis of high-resolution radio data
shows that asymmetries in the brightness distribution of the jets appear to
be well in evidence on the scale of a few kpc (Parma et al. 1994,
1996a; Laing 1994, 1996). On such small scales
therefore, this makes these objects more similar to PRG.
The polarization of the sample has been studied by Capetti et al.\ (1993, hereafter referred as Paper I) using the VLA data at 20 cm. Despite the fact that only one frequency was available, Parma et al. (1993) were able, making a number of assumptions, to investigate the asymmetry in polarization for LLRG. They concluded that, as in PRG, an asymmetry in polarization is visible in LLRG between the lobe with the jet and that with the counter-jet. This is an important result and deserves to be further investigated by a proper study of the depolarization using at least two frequencies.
Because of this, we have observed the B2 sample presented in Paper I at 6 cm with the VLA. In this paper we present the results of these observations. The estimated polarization and fractional polarization obtained at 6 cm will then be compared with those previously presented 20 cm data to investigate the integrated properties of polarization and depolarization for different types of radio galaxies. Moreover, this new set of data will allow us to do a detailed study of the depolarization in the sources with double lobes and jet and counter-jet morphology. This is discussed in a companion paper (Morganti et al. 1996).