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1. Introduction

Quasars were first discovered through the optical identification of radio sources. Optical searches, based on different criteria from multicolor selection to grism searches, have produced a large number of optically selected quasar and have shown that radio loud quasars are really a fraction of the quasar population. However, optical multicolor searches have their own problems in completeness if, for instance, a large fraction of obscured quasars exists (Webster et al. 1995). In principle radio quasars samples do not suffer from a bias of this kind and, therefore, studying their color distribution, they should provide a direct test on the number of obscured quasars.

In the last years, a renewed interest is also raising in radio samples selected at low frequency because they provide an effective way to test the Unified Schemes (US) for extragalactic radiosources (Barthel 1989) through, for instance, a direct comparison of the observed angular sizes of radiogalaxies and quasars in the same redshift range.

Quasar samples selected at meter-wavelength are preferable with respect to high frequency selected samples because the source selection is largely based on their lobe emission. On the contrary flat spectrum sources, predominant in high frequency selected samples, are mostly core-dominated cases where the relativistic beaming might introduce serious selection effects.

Although US are generally accepted, some authors found observational evidences which cannot be explained by this scheme and that require a more complex treatment of the AGN phenomenon. Using 3CR, MQS, 1JY samples and an earlier version of the present B3-VLA quasar sample (see Singal 1993 and references therein), Singal finds several discrepancies between the predictions of US and the observational data. For instance, the observed number of quasars versus total source number, namely the quasar fraction (tex2html_wrap_inline1342) changes with the limiting flux of the sample. In the MQS sample, Kapahi et al. (1996) show that apparent sizes of radiogalaxies and quasars have almost an equal distribution at any redshift in contradiction with the US prediction. A similar result has been found by Blundell et al. (1996), using 3CR, 5C, 6C and 8C samples.

In a future paper, using all the available parameters of radiogalaxies and quasars of the B3-VLA sample (apparent diameters, flux, power, redshifts), we will verify the predictions of the US.

In Sect. 2 we present the final QSO sample based on spectroscopic observations reported in Vigotti et al. (1990), Lahulla et al. (1991) and further observations of the remaining candidates described in Sect. 3 of the present paper. Section 4 contains comments on some individual sources, and finally Sect. 5 summarizes the present data.


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