BL Lacertae objects are hard to find. This is simply due to one of their defining features: the almost complete lack of emission lines. As a result, contrary to most other astronomical sources, only a few objects of this class have been discovered at optical frequencies. About 95 per cent of known BL Lacertae objects have in fact been discovered in the radio or X-ray band, where they can be more easily recognized thanks to other distinguishing properties such as flat radio spectra and a distinctive multifrequency energy distribution. BL Lacertae objects are also intrinsically rare and constitute only a few per cent of the known population of active galactic nuclei (AGN).
The need for an up-to-date BL Lac catalogue came for a practical reason. We were studying the AGN content of the WGA catalogue (White, Giommi & Angelini 1994), a large catalogue of X-ray sources generated from all the ROSAT PSPC pointed observations. We wanted to extract from it all known BL Lacertae objects to analyse their X-ray properties, but, when we started using the Véron-Cetty & Véron (1993a) and Hewitt & Burbidge (1993) catalogues, we realized that the number of BL Lacs had recently increased in such a way as to require a new compilation. Also, the criteria according to which objects had been called BL Lacs in previous catalogues were highly inhomogeneous.
We then put together a list of BL Lacertae objects, taking a novel approach. Instead of assembling all objects ever called BL Lacs in the literature, we started from the (by now quite numerous) complete samples, adding at the end additional objects from existing catalogues and the literature. By complete sample here we mean an homogeneous set of sources detected in a statistically well-defined and completely identified survey (although in some cases the identification process is not yet 100 per cent complete). In most cases statistically well-defined means flux limited in one or more energy bands.
The result of this effort is a catalogue of 233 BL Lacs, i.e. significantly larger than the Véron-Cetty & Véron (171 BL Lacs) and Hewitt & Burbidge (90 BL Lacs) catalogues but above all, we believe, based on more homogeneous criteria. Most samples, in fact, adopt a classification based on equivalent width W, 5 Å being the dividing line between BL Lacs and quasars. Although there are undoubtedly borderline objects, in which emission lines appear when the continuum is in a low state, this value seems to separate quite well the two classes (see discussion in Urry & Padovani 1995).
The structure of the paper is as follows: in Section 2 we describe the catalogue and in particular the samples on which it is mostly based, while in Section 3 we present some of the statistical properties of the catalogue.