Periodicity in the BL Lac object OJ287

Short-term variations in luminosity provide the best upper limits for the sizes of active galactic nuclei. Periodicity provides further information on the geometrical, dynamical and physical models of these regions. The best candidates for the study of such variations are the so-called blazars (optically violently variable (OVV) objects and BL Lac objects). The authors' photometric observations of OJ287 reveal periodic variability in timescales of a few minutes. It is concluded that the results are compatible with the presence of 'hotspots' in a thick accretion disk spiraling into a Kerr's supermassive black hole and that the central source is seen nearly pole-on. (author)