Improved limits on the tensor-to-scalar ratio using BICEP and Planck
  We present constraints on the tensor-to-scalar ratio r using a combination of
BICEP/Keck 2018 and Planck PR4 data allowing us to fit for r consistently with
the six parameters of the $\Lambda$CDM model without fixing any of them. In
particular, we are able to derive a constraint on the reionization optical
depth $\tau$ and thus propagate its uncertainty onto the posterior distribution
for r. While Planck sensitivity to r is no longer comparable with ground-based
measurements, combining Planck with BK18 and BAO gives results consistent with
r = 0 and tightens the constraint to r < 0.032.
