  Kaons and protons carry large parts of two conserved quantities, strangeness
and baryon number. It is argued that their correlation and thus also
fluctuations are sensitive to conditions prevailing at the anticipated
parton-hadron phase boundary. Fluctuations of the $(\mathrm{K}^+ +
\mathrm{K}^-)/(\mathrm{p}+\bar{\mathrm{p}})$ and $\mathrm{K}^+/\mathrm{p}$
ratios have been measured for the first time by NA49 in central Pb+Pb
collisions at 5 SPS energies between $\sqrt{s_{NN}}$= 6.3 GeV and 17.3 GeV.
Both ratios exhibit a change of sign in $\sigma_{\mathrm{dyn}}$, a measure of
non-statistical fluctuations, around $\sqrt{s_{NN}}$ = 8 GeV. Below this
energy, $\sigma_{\mathrm{dyn}}$ is positive, indicating higher fluctuation
compared to a mixed event background sample, while for higher energies,
$\sigma_{\mathrm{dyn}}$ is negative, indicating correlated emission of kaons
and protons. The results are compared to UrQMD calculations which which give a
good description at the higher SPS energies, but fail to reproduce the
transition to positive values.
