The earliest O-type eclipsing binary in the Small Magellanic Cloud, AzV
  476: A comprehensive analysis reveals surprisingly low stellar masses
  Massive stars at low metallicity are among the main feedback agents in the
early Universe and in present-day star forming galaxies. When in binaries,
these stars are potential progenitors of gravitational-wave events. Knowledge
of stellar masses is a prerequisite to understanding evolution and feedback of
low-metallicity massive stars. Using abundant spectroscopic and photometric
measurements of an outstandingly bright eclipsing binary, we compare its
dynamic, spectroscopic, and evolutionary mass estimates and develop a binary
evolution scenario. We comprehensively studied the eclipsing binary system, AzV
476, in the Small Magellanic Cloud. The light curve and radial velocities were
analyzed to obtain the orbital parameters. The photometric and spectroscopic
data in the UV and optical were analyzed using the Potsdam Wolf-Rayet model
atmospheres. The obtained results are interpreted using binary-evolution
tracks. AzV 476 consists of an O4IV-III((f))p primary and an O9.5:Vn secondary.
Both components have similar current masses (~20 M$_{\odot}$) obtained from
both the orbital and spectroscopic analysis. The wind mass-loss rate of
log($\dot{M}$/(M$_{\odot}$/yr))=-6.2 of the primary is a factor of ten higher
than a recent empirical prescription for single O stars in the SMC. Only
close-binary evolution with mass transfer can reproduce the current stellar and
orbital parameters. The binary evolutionary model reveals that the primary has
lost about half of its initial mass and is already core helium burning. Our
comprehensive analysis of AzV 476 yields a consistent set of parameters and
suggests previous case B mass transfer. The derived stellar masses agree within
their uncertainties. The moderate masses of AzV 476 underline the scarcity of
bright massive stars in the SMC. The core helium burning nature of the primary
indicates that stripped stars might be hidden among OB-type populations.
