  Graham et al. (2015) discovered a supermassive black hole binary (SMBHB)
candidate and identified the detected 5.2 yr period of the optical variability
as the orbital period of the binary. Hydrodynamical simulations predict
multiple periodic components for the variability of SMBHBs, thus raising the
possibility that the true period of the binary is different from 5.2 yr. We
analyse the periodogram of PG1302 and find no compelling evidence for
additional peaks. We derive upper limits on any additional periodic modulations
in the available data, by modeling the light-curve as the sum of red noise and
the known 5.2 yr periodic component, and injecting additional sinusoidal
signals. We find that, with the current data, we would be able to detect with
high significance (false alarm probability <1%) secondary periodic terms with
periods in the range predicted by the simulations, if the amplitude of the
variability was at least ~0.07 mag (compared to 0.14 mag for the main peak). A
three-year follow-up monitoring campaign with weekly observations can increase
the sensitivity for detecting secondary peaks ~50%, and would allow a more
robust test of predictions from hydrodynamical simulations.
