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Compensated isocurvature perturbations (CIPs) are primordial fluctuationsthat balance baryon and dark-matter isocurvature to leave the total matterdensity unperturbed. The effects of CIPs on the cosmic microwave background(CMB) anisotropies are similar to those produced by weak lensing of the CMB:smoothing of the power spectrum, and generation of non-Gaussian features.Previous work considered the CIP effects on the CMB power-spectrum butneglected to include the CIP effects on estimates of the lensing potentialpower spectrum (though its contribution to the non-Gaussian, connected, part ofthe CMB trispectrum). Here, the CIP contribution to the standard estimator forthe lensing potential power-spectrum is derived, and along with the CIPcontributions to the CMB power-spectrum, Planck data is used to place limits onthe root-mean-square CIP fluctuations on CMB scales, $\Delta_{\rm rms}^2(R_{\rmCMB})$. The resulting constraint of $\Delta_{\rm rms}^2(R_{\rm CMB}) < 4.3\times 10^{-3}$ using this new technique improves on past work by a factor of$\sim 3$. We find that for Planck data our constraints almost reach thesensitivity of the optimal CIP estimator. The method presented here iscurrently the most sensitive probe of the amplitude of a scale-invariant CIPpower spectrum placing an upper limit of $A_{\rm CIP}< 0.017$ at 95% CL. Futuremeasurements of the large-scale CMB lensing potential power spectrum couldprobe CIP amplitudes as low as $\Delta_{\rm rms}^2(R_{\rm CMB}) = 8 \times10^{-5}$ ($A_{\rm CIP} = 3.2 \times 10^{-4}$).

Baryons still trace dark matter: probing CMB lensing maps for hidden isocurvature