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tokens: to see if the hint for high redshift contributions to is sharpened by future CMB polarization data. We eagerly await upcoming results from the new Planck HFI large-scale polarization data which is still proprietary to see if these observations strengthen the hint seen in the LFI data. Alternatively, these may reflect remaining systematics in the LFI data that do not also apply to the HFI data. Based on Fig. 5 , we expect that a future CV-limited experiment should be able to confirm or refute the high redshift contribution to , although we postpone a more detailed investigation of these prospects to future work. It will be extremely challenging to test the hint for non-negligible ionization fractions at z 15 by other means. The best alternative is likely redshifted 21cm surveys but these face challenges at high redshift owing to the bright galactic emission at the frequencies of interest. However, these surveys may find that the average neutral fraction rises more slowly towards high redshift than expected in models with Pop-II stars alone (see Fig. 1 ). There may be differences in the sizes of the ionized regions in these scenarios as well, which may provide a potential signature (Koh Wise 2016 ). However, the possibility of a significant contribution from Pop-III stars at high redshift may also somewhat complicate the interpretation of the 21cm power spectrum measurements (e.g. Fialkov etal. 2013 ). Another possibility may be to extract signatures of early phases of patchy reionization using the Kinetic SunyaevZeldovich effect, but these will have to push to smaller angular scales, which is challenging given foreground contamination. An extended self-regulated phase likely evades current constraints (Zahn etal. 2012 ; George etal. 2015 ) on the duration of reionization (Park etal. 2013 ). Finally, it is worth commenting on the implications of our findings for the goal of determining from the ionization history inferred from redshifted 21cm observations (Liu etal. 2016 ). This is an appealing idea, because the optical depth is an important nuisance parameter that can limit, for instance, inferences regarding the sum of the neutrino masses from upcoming CMB lensing measurements. However, this prospect becomes difficult if there are significant high redshift contributions to the optical depth, as hinted at in the Planck LFI data, which will be hard to extract from upcoming redshifted 21cm surveys. We hope that this work will encourage reionization modellers to adopt this PC analysis methodology of Hu Holder ( 2003 ) and effective likelihood of Heinrich etal. ( 2016 ). Here we have considered rather simple models for the ionization history in an effort to explore the broad-brush implications of the Planck data, but it would be interesting to explore more detailed reionization models and to combine the full information content of the CMB data with other reionization observables. It will also be interesting to explore whether dark matter