The catalogue includes all objects from the BL Lac samples known to us at the time of writing (1995 June) plus objects listed in the Véron-Cetty & Véron (1993a) and Hewitt & Burbidge (1993) catalogues as follows: objects common to the two catalogues were included under the label ``miscellaneous''; objects belonging to only one of the two were conservatively labelled ``candidate''. This last group includes also BL Lac candidates belonging to complete samples or found in the literature. We have excluded from our list objects originally classified as BL Lacs or BL Lac candidates but which were later shown to have strong lines: examples are 2201+044, included in the HEAO-1 sample by Laurent-Muehleisen et al. (1993) but recently shown to be a Seyfert 1 galaxy (Véron-Cetty & Véron 1993b), and 1214+1753, a BL Lac candidate in Foltz et al. (1987) but a broad absorption line QSO in Stocke et al. (1992).
The catalogue, which is ``complete'', to the best of our knowledge, as far as BL Lacs in samples and miscellaneous objects are concerned, is presented in Table 1. Column 1 gives the most common name(s), columns 2 and 3 the J2000 positions for each object. Columns 4, 5 and 6 give the redshift, V magnitude and radio flux at 5 GHz, while column 7 contains the references for these quantities. Finally, in column 8 we give references to X-ray data while in column 9 we report the sample(s) to which the object belongs or the ``miscellaneous'' or ``candidate'' classification (the latter followed, if applicable, by the name of the sample to which the object belongs). The data generally come ``as given'' by the paper describing the sample or from the catalogues. When radio fluxes were not provided, we searched available 5-GHz radio catalogues at northern and southern declinations (Becker, White & Edwards 1991; Wright et al. 1994; Griffith et al. 1994, 1995), which at present reach ~ 20 - 40 mJy. It can then be safely assumed that the 13 BL Lacs in this catalogue lacking radio data have 5-GHz radio fluxes below these limits. Positions are usually good to within a few arcseconds for the radio- and optically-selected sources. A similar accuracy is reached also by the X-ray-selected sources in the Einstein Imaging Proportional Counter (IPC) Slew survey, for most of which precise radio positions are given in Perlman et al. (1995b), in the Einstein Observatory Extended Medium Sensitivity Survey (EMSS), whose optical coordinates have been taken from Maccacaro et al. (1994), and in the ROSAT all-sky survey (RASS), whose BL Lacs have optical coordinates (Bade, Fink & Engels 1994). We also present here previously unpublished radio positions, good to within 1 - 2 arcsec, for the following EXOSAT BL Lacs: EXO0706.1+5913, EXO0811.2+2949, EXO1004.0+3509, EXO1118.0+4228, EXO1146.9+2455, and EXO1811.7+3143. In a few cases accurate radio positions were obtained from the NASA/IPAC Extragalactic Database (NED). In summary, coordinates are uncertain up to a few tens of arcseconds only for four EXOSAT BL Lacs (EXO0044.4+2001, EXO0423.4-0840, EXO0556.4-3838, and EXO1415.6+2557), one HEAO-1 BL Lac (1H 0829+090, plus 1H 1914-194, for which coordinates are not available), and the five new RASS BL Lacs published by Brinkmann et al. (1995). In cases when an object belongs to more than one sample, the most accurate coordinates are quoted.
One of the characteristics of BL Lacs is their strong variability, which generally increases with frequency. While this is not a problem in the radio band where large-amplitude variability is rare (e.g. Miller & Wiita 1991), V magnitudes should be taken with care. As regards the 1-Jy BL Lacs, however, we provide values which should be quite representative of the ``typical'' state of the objects, from Padovani (1992). The reader is referred to that paper for a description of the derivation of these magnitudes and for the appropriate references. We also note that optical monitoring data for many BL Lacs are available in the literature (e.g. Pica et al. 1988; Webb et al. 1988; Falomo, Scarpa & Bersanelli 1994).
Given that a number of additional complications are present in the X-ray data (different instruments and bands, uncertainties in the spectral indices, photoelectric absorption), we do not provide X-ray fluxes. We prefer instead to give at least one reference, so that X-ray data can be easily retrieved. X-ray references have been compiled as follows: our main sources were the catalogue of Della Ceca et al. (1990), a compilation of X-ray data up to 1986, and the catalogue of X-ray spectra of Ciliegi, Bassani & Caroli (1993), complete up to the end of 1991. We also refer to the recent WGA catalogue of ROSAT PSPC sources (White et al. 1994), while in the case of X-ray-selected objects we refer to the original papers describing the sample. Finally, for those sources not included in the above-mentioned catalogues, we first performed a literature search and then searched the Einstein IPC and EXOSAT databases. At the end of this process, 30 objects (or about 13 per cent of the catalogue) had no X-ray references. Considering only confirmed BL Lacs this number goes down to 10 (or about 5 per cent of the confirmed objects).
The catalogue breaks up into the following sub-classes: confirmed BL Lacs in samples (159 objects or 68 per cent of the catalogue), miscellaneous BL Lacs (24 objects or 10 per cent of the catalogue), and BL Lac candidates or objects whose BL Lac classification is still uncertain (50 objects or 22 per cent of the catalogue), for a total of 233 objects.
We now describe the various samples and classes, grouping them into wavelength bands.