  Compact binaries with unequal masses and whose orbits are not aligned with
the observer's line of sight are excellent probes of gravitational radiation
beyond the quadrupole approximation. Among the compact binaries observed so
far, strong evidence of octupolar modes is seen in GW190412 and GW190814, two
binary black holes observed during the first half of the third observing run of
LIGO/Virgo observatories. These two events, therefore, provide a unique
opportunity to test the consistency of the octupolar modes with the predictions
of general relativity (GR). In the post-Newtonian (PN) approximation to GR, the
gravitational-wave phasing has known dependencies on different radiative
multipole moments, including the mass octupole. This permits the use of
publicly released posteriors of the PN phase deformation parameters for placing
constraints on the deformations to the different PN components of the radiative
mass octupole denoted by $\delta \mu_{3n}$. Combining the posteriors on $\delta
\mu_{3n}$ from these two events, we deduce a joint bound (at 90% credibility)
on the first three PN order terms in the radiative octupoles to be $\delta
\mu_{30}=-0.07^{+0.11}_{-0.12}$, $\delta \mu_{32}=0.48^{+0.93}_{-1.15}$, and
$\delta \mu_{33}=-0.32^{+1.67}_{-0.62}$, consistent with GR predictions. Among
these, the measurement of $\delta \mu_{33}$ for the first time confirms the
well-known octupolar tail contribution, a novel nonlinear effect due to the
scattering of the octupolar radiation by the background spacetime, is
consistent with the predictions of GR. Detection of similar systems in the
future observing runs should further tighten these constraints.
