Periodic component in the variations of brightness and polarization of BL Lacertae

We have analyzed a prolonged (22 yr) series of photometric and polarimetric optical observations of BL Lac. We have found evidence for periodicity of the same period (P = 308 days) for variations in both total flux and relative Stokes parameter q in the second half of the data (1980-1991). This is the first time that the same periodic component-over more than 10 cycles-has been found in two observational sets obtained independently. Our results show that, although periodic behavior may exist over a long time interval, periodic variations switch on and off. This might explain discrepant results obtained by different authors who have tested for periodicity different parts of the light curve of the same object.