  The recent electron recoil excess observed by XENON1T has a possible
interpretation in terms of solar axions coupled to electrons. If such axions
are still relativistic at recombination they would also leave a cosmic imprint
in the form of an additional radiation component, parameterized by an effective
neutrino number $\Delta N_\text{eff}$. We explore minimal scenarios with a
detectable signal in future CMB surveys: axions coupled democratically to all
fermions, axion-electron coupling generated radiatively, the DFSZ framework for
the QCD axion. The predicted $\Delta N_\text{eff}$ is larger than $0.03-0.04$
for all cases, close to the $2\sigma$ forecasted sensitivity of CMB-S4
experiments. This opens the possibility of testing with cosmological
observations the solar axion interpretation of the XENON1T excess.
