  We report the results of a joint analysis of data from BICEP2/Keck Array and
Planck. BICEP2 and Keck Array have observed the same approximately 400 deg$^2$
patch of sky centered on RA 0h, Dec. $-57.5\deg$. The combined maps reach a
depth of 57 nK deg in Stokes $Q$ and $U$ in a band centered at 150 GHz. Planck
has observed the full sky in polarization at seven frequencies from 30 to 353
GHz, but much less deeply in any given region (1.2 $\mu$K deg in $Q$ and $U$ at
143 GHz). We detect 150$\times$353 cross-correlation in $B$-modes at high
significance. We fit the single- and cross-frequency power spectra at
frequencies $\geq 150$ GHz to a lensed-$\Lambda$CDM model that includes dust
and a possible contribution from inflationary gravitational waves (as
parameterized by the tensor-to-scalar ratio $r$), using a prior on the
frequency spectral behavior of polarized dust emission from previous \planck\
analysis of other regions of the sky. We find strong evidence for dust and no
statistically significant evidence for tensor modes. We probe various model
variations and extensions, including adding a synchrotron component in
combination with lower frequency data, and find that these make little
difference to the $r$ constraint. Finally we present an alternative analysis
which is similar to a map-based cleaning of the dust contribution, and show
that this gives similar constraints. The final result is expressed as a
likelihood curve for $r$, and yields an upper limit $r_{0.05}<0.12$ at 95%
confidence. Marginalizing over dust and $r$, lensing $B$-modes are detected at
$7.0\,\sigma$ significance.
