Metastable states of an exotic atom He+(p) over bar are reviewed. We presen
t theoretical calculation of transition energies between metastable states
that includes relativistic and quantum electrodinamic corrections. The accu
racy of a theoretical value for the (36,35) --> (34,33) transition, which i
s considered as a candidate for the high-precision two-photon induced trans
ition measurement in the ASACUSA experiment at CERN, is estimated to be abo
ut 2 ppb.