Optical variability and periodicity analysis for blazars. I. Light curves for radio-selected BL Lacertae objects

Photometric observations of AGNs are important to construct light curves an d to study variability behavior over different timescales. In this paper, t he historical optical data compiled from the published literature are prese nted in tabular form as an optical database for 28 radio-selected BL Lacert ae objects. Maximum optical variations are found and compared with the infr ared ones. The faintest, brightest, and averaged magnitudes are presented f or individual object. The variation is found correlated with polarization. The light curves are shown in figures along with the relation between color index and magnitude. For the color-magnitude relation, two objects, 0735+1 78 and 0537-441, show evidence that the color index increases with magnitud e, indicating that the spectrum flattens when the source brightens, while t hree objects (0219+428, 1514-241, 2200+420) show the opposite behavior; an object, 0235+164, shows a brightness-dependent color index-magnitude correl ation, while the rest of the objects do not show any clear tendency.