[en] Using two equipment, the following experiments have been made: High-precision measurements of the wavelengths of transitions between five pairs of metastable and short-lived antiprotonic states in low-density (0.1-5 mbar) ⁴He and ³He. The measurements at low densities (<1 mbar) revealed that the average lifetime of antiprotons (which can be deduced from the average decay rate of the delayed annihilation time spectra) increases to ∼5 microseconds compared to 3.5 microseconds at higher densities (>100 mbar). Four new laser resonant transitions were discovered between antiprotonic states, one in ⁴He and three in ³He. (R.P.)