Sandrinelli, A.A.SandrinelliCOVINO, StefanoStefanoCOVINOTreves, A.A.TrevesHolgado, A. M.A. M.HolgadoSesana, A.A.SesanaLindfors, E.E.LindforsRamazani, V. F.V. F.Ramazani2020-10-232020-10-2320180004-6361http://hdl.handle.net/20.500.12386/27965We review the reports of possible year-long quasi-periodicities of BL Lac objects in the γ-ray and optical bands, and present a homogeneous time analysis of the light curves of PKS2155-304, PG1553+113, and BL Lac. Based on results from a survey covering the entire Fermi γ-ray sky we have estimated the fraction of possible quasi-periodic BL Lac objects. We compared the cyclical behaviour in BL Lac objects with that derived from the search of possible optical periodicities in quasars, and find that at z ≲ 1 the cosmic density of quasi-periodic BL Lac objects is larger than that of quasi-periodic quasars. If the BL Lac quasi-periodicities were due to a supermassive binary black hole (SBBH) scenario, there could be a tension with the upper limits on the gravitational wave background measured by the pulsar timing array. The argument clearly indicates the difficulties of generally associating quasi-periodicities of BL Lac objects with SBBHs.STAMPAenArticle10.1051/0004-6361/2017325502-s2.0-85054092581000439936700007https://www.aanda.org/articles/aa/abs/2018/07/aa32550-17/aa32550-17.html2018A&A...615A.118SFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_10 High energy and particle astronomy – X-rays, cosmic rays, gamma rays, neutrinos