  The strong interaction of antikaons with nucleons and nuclei in the
low-energy regime represents an active research field connected intrinsically
with few-body physics. There are important open questions like the question of
antikaon nuclear bound states. A unique and rather direct experimental access
to the antikaon-nucleon scattering lengths is provided by precision X-ray
spectroscopy of transitions in low-lying states of light kaonic atoms like
kaonic hydrogen isotopes. In the SIDDHARTA experiment at the electron-positron
collider DAFNE of LNF-INFN we measured the most precise values of the strong
interaction observables, i.e. the strong interaction on the 1s ground state of
the electromagnetically bound kaonic hydrogen atom leading to a hadronic shift
and a hadronic broadening of the 1s state. The SIDDHARTA result triggered new
theoretical work which achieved major progress in the understanding of the
low-energy strong interaction with strangeness. Antikaon-nucleon scattering
lengths have been calculated constrained by the SIDDHARTA data on kaonic
hydrogen. For the extraction of the isospin-dependent scattering lengths a
measurement of the hadronic shift and width of kaonic deuterium is necessary.
Therefore, new X-ray studies with the focus on kaonic deuterium are in
preparation (SIDDHARTA2). Many improvements in the experimental setup will
allow to measure kaonic deuterium which is challenging due to the anticipated
low X-ray yield. Especially important are the data on the X-ray yields of
kaonic deuterium extracted from a exploratory experiment within SIDDHARTA.
