A novel CMB polarization likelihood package for large angular scales
  built from combined WMAP and Planck LFI legacy maps
  We present a CMB large-scale polarization dataset obtained by combining WMAP
Ka, Q and V with Planck 70 GHz maps. We employ the legacy frequency maps
released by the WMAP and Planck collaborations and perform our own Galactic
foreground mitigation technique, which relies on Planck 353 GHz for polarized
dust and on Planck 30 GHz and WMAP K for polarized synchrotron. We derive a
single, optimally-noise-weighted, low-residual-foreground map and the
accompanying noise covariance matrix. These are shown, through $\chi^2$
analysis, to be robust over an ample collection of Galactic masks. We use this
dataset, along with the Planck legacy Commander temperature solution, to build
a pixel-based low-resolution CMB likelihood package, whose robustness we test
extensively with the aid of simulations, finding excellent consistency. Using
this likelihood package alone, we constrain the optical depth to reionazation
$\tau=0.069^{+0.011}_{-0.012}$ at $68\%$ C.L., on 54\% of the sky. Adding the
Planck high-$\ell$ temperature and polarization legacy likelihood, the Planck
lensing likelihood and BAO observations we find
$\tau=0.0714_{-0.0096}^{+0.0087}$ in a full $\Lambda$CDM exploration. The
latter bounds are slightly less constraining than those obtained employing
\Planck\ HFI CMB data for large angle polarization, that only include EE
correlations. Our bounds are based on a largely independent dataset that does
include also TE correlations. They are generally well compatible with Planck
HFI preferring slightly higher values of $\tau$. We make the low-resolution
Planck and WMAP joint dataset publicly available along with the accompanying
likelihood code.
