1 Introduction

BL Lacertae objects (BL Lacs) are an extreme form of active galactic nuclei (AGNs) characterized by rapid variability, high polarization, a compact radio structure, and weak or absent emission lines. As BL Lacs have numerous characteristics in common with flat-spectrum radio quasars (FSRQs), they are classified together as "blazars''. Their traits are generally interpreted as being the signature of a relativistic jet pointing towards us. For a better understanding of the phenomenon that creates such extreme features, we need to discriminate among the theoretical models that try to explain them. To make this study possible, a big observational effort is required. Optical observations offer a wealth of information on the variability of BL Lacs. From such data, physical conditions of the emission region can be inferred.

As many important answers about the mechanisms that produce the emission at the various wavelengths are still to be given, simultaneous observations of these objects in different bands can help in making a better idea about what they really are, by revealing some possible correlations. The launch of satellites for astronomical researches made this kind of study possible. In particular, the collection of simultaneous optical and high-energy data is thought to be very important, since it allows to reconstruct that part of the spectral energy distribution (SED) of the source where a transition between the synchrotron emission and the inverse-Compton one is expected.

In Raiteri et al. ([1998a]) we presented optical data on eight X-ray selected BL Lacs observed in connection with pointings of BeppoSAX. The corresponding X-ray data were published in Wolter et al. ([1998]). In the following we present optical data for five X-ray selected BL Lacs observed in 1996-1999 in periods including the BeppoSAX observations of these sources.