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Periodic quasars are candidates for binary supermassive black holes (BSBHs)efficiently emitting low frequency gravitational waves. Recently, $\sim$150candidates were identified from optical synoptic surveys. However, they may befalse positives caused by stochastic quasar variability given the few cyclescovered (typically 1.5). To independently test the binary hypothesis, we searchfor evidence of truncated or gapped circumbinary accretion discs (CBDs) intheir spectral energy distributions (SEDs). Our work is motivated by CBDsimulations that predict flux deficits as cutoffs from central cavities openedby secondaries or notches from minidiscs around both BHs. We find thatcandidate periodic quasars show SEDs similar to those of control quasarsmatched in redshift and luminosity. While seven of 138 candidates show a bluecutoff in the IR-optical-UV SED, six of which may represent CBDs with centralcavities, the red SED fraction is similar to that in control quasars,suggesting no correlation between periodicity and SED anomaly. Alternatively,dust reddening may cause red SEDs. The fraction of extremely radio-loudquasars, e.g., blazars (with $R>100$), is tentatively higher than that incontrol quasars (at 2.5$\sigma$). Our results suggest that, assuming mostperiodic candidates are robust, IR-optical-UV SEDs of CBDs are similar to thoseof accretion discs of single BHs, if the periodicity is driven by BSBHs; thehigher blazar fraction may signal precessing radio jets. Alternatively, mostcurrent candidate periodic quasars identified from few-cycle light curves maybe false positives. Their tentatively higher blazar fraction and lowerEddington ratios may both be caused by selection biases.

Spectral Energy Distributions of Candidate Periodically-Variable Quasars: Testing the Binary Black Hole Hypothesis