1 Introduction

A great effort of the recent research on Active Galactic Nuclei (AGN) has been directed to the development of "Unified Schemes'': a framework wherein the observational properties of different classes of AGN can be explained as intrinsically similar objects seen at different orientation angles to the line of sight (see Urry & Padovani 1995 and references therein).

In this context, it is now widely accepted that the observed properties of BL Lac objects are largely due to a relativistic jet pointing in the direction of the observer (the Beaming Model; Blandford & Rees 1978), implying the existence of a class of radio sources (hereafter the parent population) intrinsically identical to BL Lacs, but with the jets oriented at large angles with respect to the line of sight.

The nature of the parent population of the BL Lacs has been the subject of several investigations in the past. While some authors confirm that the majority of them are low luminosity edge darkened FR I radio galaxies seen along their radio axis (see Urry & Padovani 1995 and references therein), others suggest that the diffuse radio emission detected around high redshift BL Lacs is more consistent with FR II rather than FR I radio galaxies (see e.g. Kollgaard et al. 1992; Murphy et al. 1993).

A straightforward outcome of the beaming model is that all the properties not depending on orientation should be shared by the BL Lac objects and their parent population.

To address this problem we planned to compare the extended radio luminosity of the radio selected 1 Jy BL Lac sample (Stickel et al. 1991) with that of FR I and FR II radio galaxies. In fact, while the morphology is distorted by projection effects, and a morphological classification could be uncertain, the luminosity of the unbeamed emission is suitable for a direct comparison with the extended emission found in the candidate parent population.

Of the 34 BL Lac objects belonging to the 1 Jy sample we have selected the 28 objects without deep radio images at arcsecond scale resolution in the literature, and/or at the arcminute scale. Stickel et al. (1994) added three sources to their original sample: 0218+357, 2029+121 and 2150+173. In particular 0218+357 is a well known gravitational lens (O'Dea et al. 1992; Patnaik et al. 1993, 1995). Since our work started when these three additional sources were not included, and furthermore, in the literature the "1 Jy sample of BL Lac sources'' refers to the 34 objects in Stickel et al. (1991), we will not consider the forementioned additional sources.

Here we present data for 28 objects observed with the VLA and/or the WSRT; preliminary results on extended emission and polarization properties for a small fraction of these objects were presented in Stanghellini et al. (1997). In this paper we show the images and briefly discuss the results obtained from our observations. In a forthcoming paper we will give a full discussion of the results. Throughout this paper we use H₀ = 50 km s^-1 Mpc^-1 and q₀ = 0.5.