Axions currently provide the most compelling solution to the strong CP problem .
These particles may be copiously produced in the early universe , including via thermal processes .
Therefore , relic axions constitute a hot dark matter component and their masses are strongly degenerate with those of the three active neutrinos , as they leave identical signatures in the different cosmological observables .
In addition , thermal axions , while still relativistic states , also contribute to the relativistic degrees of freedom , parameterised via N _ { \textrm { eff } } .
We present the cosmological bounds on the relic axion and neutrino masses , exploiting the full Planck mission data , which include polarization measurements .
In the mixed hot dark matter scenario explored here , we find the tightest and more robust constraint to date on the sum of the three active neutrino masses , \sum m _ { \nu } < 0.136 eV at 95 \% CL , obtained in the well-known linear perturbation regime .
The Planck Sunyaev-Zeldovich cluster number count data further tightens this bound , providing a 95 \% CL upper limit of \sum m _ { \nu } < 0.126 eV in this very same mixed hot dark matter model , a value which is very close to the expectations in the inverted hierarchical neutrino mass scenario .
Using this same combination of data sets we find the most stringent bound to date on the thermal axion mass , m _ { a } < 0.529 eV at 95 \% CL .