  We present the first measurements of femtoscopic correlations between the
K$^0_{\rm S}$ and K$^{\rm \pm}$ particles in pp collisions at $\sqrt{s}=7$ TeV
measured by the ALICE experiment. The observed femtoscopic correlations are
consistent with final-state interactions proceeding solely via the $a_0(980)$
resonance. The extracted kaon source radius and correlation strength parameters
for K$^0_{\rm S}$K$^{\rm -}$ are found to be equal within the experimental
uncertainties to those for K$^0_{\rm S}$K$^{\rm +}$. Results of the present
study are compared with those from identical-kaon femtoscopic studies also
performed with pp collisions at $\sqrt{s}=7$ TeV by ALICE and with a K$^0_{\rm
S}$K$^{\rm \pm}$ measurement in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$
TeV. Combined with the Pb-Pb results, our pp analysis is found to be compatible
with the interpretation of the $a_0(980)$ having a tetraquark structure instead
of that of a diquark.
