  Galaxy clusters are assembled via merging of smaller structures, in a process
that generates shocks and turbulence in the intra cluster medium and produces
radio diffuse emission in the form of halos and relics. The cluster pair
A399-A401 represents a special case: both clusters host a radio halo. Recent
Low Frequency Array (LOFAR) observations at 140 MHz revealed the presence of a
radio bridge connecting the two clusters along with two relic candidates. These
relics include one South of A399 and the other in between the two clusters, in
proximity of a shock front detected in X-ray observations. In this paper we
present observations of the A399-A401 cluster pair at 1.7, 1.4, 1.2 GHz and 346
MHz from the Westerbork Synthesis Radio Telescope (WSRT). We detect the radio
halo in the A399 cluster at 346 MHz, extending up to $\sim 650$ kpc and with a
$125 \pm 6$ mJy flux density. Its spectral index between 140 MHz and 346 MHz is
$\alpha = 1.75 \pm 0.14$. The two candidate relics are also seen at 346 MHz and
we determine their spectral indices to be $\alpha = 1.10 \pm 0.14$ and $\alpha
= 1.46 \pm 0.14$. The low surface brightness bridge connecting the two clusters
is below the noise level at 346 MHz, therefore we constrain the bridge average
spectral index to be steep, i.e. $\alpha > 1.5$ at 95% confidence level. This
result favours the scenario where dynamically-induced turbulence is a viable
mechanism to reaccelerate a population of mildly relativistic particles and
amplify magnetic fields on scales of a few Mpcs.
  Key words: galaxies: clusters: general - galaxies: clusters: individual:
Abell 399 - radio continuum: general
