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