  We investigate the nature of the BL Lac candidates in the northern part of
the 2BL sample (\cite{Londish02}) searching for optical variability by means of
$R_C$ band photometry with the Asiago 1.8 and Loiano 1.5 meter telescopes
during years 2002, 2003 and 2004. We also made aperture photometry of the
candidates on the plates available on the web from the Palomar and UK Schmidt
telescopes. No significant variability was detected for the majority of the
objects without detected radio and/or X-ray emissions, while clear variability
was found for the few sources of the sample with strong radio and/or X-ray
emission, which is a constant feature of the "classical" BL Lacs. Some
considerations on the nature of the 2BL sources are made, taking into account
their most recent data available in proper motions catalogues (SuperCOSMOS,
USNO-B1) and in the Sloan Digital Sky Survey.
