  Radiative returns to the Z resonance (Zgamma events) are used to determine
the LEP2 centre-of-mass energy from the data collected with the ALEPH detector
in 1997. The average centre-of-mass energy is measured to be:
  E_CM = 182.50 +- 0.19(stat) +- 0.08(syst) GeV
  in good agreement with the precise determination by the LEP energy working
group of 182.652 +- 0.050 GeV. If applied to the measurement of the W mass, its
precision translates into a systematic error on M_W which is smaller than the
statistical error achieved from the corresponding dataset.
