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We have measured the forbidden 2 3S1 - 2 1P1 transition at 887 nm in aquantum degenerate gas of metastable He-4 atoms confined in an optical dipoletrap. The determined transition frequency is 338 133 594.4 (0.5) MHz, fromwhich we obtain an ionization energy of the 2 1P1 state of 814 709 148.6 (0.5)MHz. This ionization energy is in disagreement by > 3 sigma with the mostaccurate quantum electrodynamics (QED) calculations available. Our measurementsalso provide a new determination of the lifetime of the 2 1P1 state of 0.551(0.004)stat (+ 0.013,- 0.000)syst ns, which is the most accurate determinationto date and in excellent agreement with theory.

High-precision spectroscopy of the forbidden 2 3S1 - 2 1P1 transition in quantum degenerate metastable helium