  We present spectroscopy and photometry of the He-rich supernova (SN) 2008ax.
The early-time spectra show prominent P-Cygni H lines, which decrease with time
and disappear completely about two months after the explosion. In the same
period He I lines become the most prominent spectral features. SN 2008ax
displays the ordinary spectral evolution of a type IIb supernova. A stringent
pre-discovery limit constrains the time of the shock breakout of SN 2008ax to
within only a few hours. Its light curve, which peaks in the B band about 20
days after the explosion, strongly resembles that of other He-rich
core-collapse supernovae. The observed evolution of SN 2008ax is consistent
with the explosion of a young Wolf-Rayet (of WNL type) star, which had retained
a thin, low-mass shell of its original H envelope. The overall characteristics
of SN 2008ax are reminiscent of those of SN 1993J, except for a likely smaller
H mass. This may account for the findings that the progenitor of SN 2008ax was
a WNL star and not a K supergiant as in the case of SN 1993J, that a prominent
early-time peak is missing in the light curve of SN 2008ax, and that Halpha is
observed at higher velocities in SN 2008ax than in SN 1993J.
