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Graham et al. (2015) have detected a 5.2 year periodic optical variability ofthe quasar PG 1302-102 at redshift $z=0.3$, which they interpret as theredshifted orbital period $(1+z)t_{\rm bin}$ of a putative supermassive blackhole binary (SMBHB). Here we consider the implications of a $3-8$ times shorterorbital period, suggested by hydrodynamical simulations of circumbinary discs(CBDs) with nearly equal--mass SMBHBs ($q\equiv M_2/M_1\gtrsim 0.3$). With thecorresponding $2-4$ times tighter binary separation, PG 1302 would beundergoing gravitational wave dominated inspiral, and serve as a proof that theBHs can be fueled and produce bright emission even in this late stage of themerger. The expected fraction of binaries with the shorter $t_{\rm bin}$, amongbright quasars, would be reduced by 1-2 orders of magnitude, compared to the5.2 year period, in better agreement with the rarity of candidates reported byGraham et al. (2015). Finally, shorter periods would imply higher binaryspeeds, possibly imprinting periodicity on the light curves from relativisticbeaming, as well as measurable relativistic effects on the Fe K$\alpha$ line.The CBD model predicts additional periodic variability on timescales of $t_{\rmbin}$ and $\approx 0.5 t_{\rm bin}$, as well as periodic variation of broadline widths and offsets relative to the narrow lines, which are consistent withthe observations. Future observations will be able to test these predictionsand hence the binary+CBD hypothesis for PG 1302.

A reduced orbital period for the supermassive black hole binary candidate in the quasar PG 1302-102?