Title: Gamma-ray and Optical Oscillations of 0716+714, MRK 421, and BL Lac,
Abstract: We examine the 2008-2016 $\gamma$-ray and optical light curves of three
bright BL Lac objects, 0716+714, MRK 421, BL Lac, which exhibit large
structured variability. We searched for periodicities by using a fully Bayesian
approach. For two out of three sources investigated no significant periodic
variability was found. In the case of BL Lac we detected a periodicity of ~ 680
days. Although the signal related to this is modest, the coincidence of the
periods in both gamma and optical bands is indicative of a physical relevance.
Considering previous literature results, possibly related $\gamma$-ray and
optical periodicities of about one year time scale are proposed in 4 bright
$\gamma$-ray blazars out of the 10 examined in detail. Comparing with results
from periodicity search of optical archives of quasars, the presence of
quasi-periodicities in blazars might be more frequent by a large factor. This
suggests the intriguing possibility that the basic conditions for their
observability are related to the relativistic jet in the observer direction,
but the overall picture remains uncertain.