  This paper describes measurements of the hyperfine structure of two
antiprotonic atoms that are planned at the Antiproton Decelerator (AD) at CERN.
The first part deals with antiprotonic helium, a three-body system of
alpha-particle, antiproton and electron that was previously studied at LEAR. A
measurement will test existing three-body calculations and may - through
comparison with these theories - determine the magnetic moment of the
antiproton more precisely than currently available, thus providing a test of
CPT invariance. The second system, antihydrogen, consisting of an antiproton
and a positron, is planned to be produced at thermal energies at the AD. A
measurement of the ground-state hyperfine splitting, which for hydrogen is one
of the most accurately measured physical quantities, will directly yield a
precise value for the magnetic moment of the antiproton, and also compare the
internal structure of proton and antiproton through the contribution of the
magnetic size of the antiproton to the ground state hyperfine splitting.
