Abstract
Context. BL Lacertae objects with an extremely low value of the peak frequency (νs ≤ 10 13 Hz) of the synchrotron component (VLBL objects) are poorly known. Aims. We selected a small sample of these candidate objects on the basis of their radio/optical flux ratio to evaluate their variability, define the low energy part of their SED and obtain a reliable estimate of their νs through a fit with a log-parabolic law. Methods. They were the object of a ground-based observational campaign in the infrared and optical bands; data from several pointings by the Swift satellite in the UV and X-ray band were also obtained for half the sample. We complemented our data with a large number of literature data. Results. These sources showed a marked flux variability, as expected for BL Lacertae objects of the low energy peaked BL Lac objects (LBL) and intermediate BL Lac objects type; all of them have a νs value around 10 13 Hz, but this is low enough only for a few sources to consider them genuine VLBL objects. We report the overall properties of individual sources and discuss some aspects of this class of rare objects, which could correspond to the transition between flat spectrum radio quasars and LBLs.
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