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Massive stars at low metallicity are among the main feedback agents in theearly Universe and in present-day star forming galaxies. When in binaries,these stars are potential progenitors of gravitational-wave events. Knowledgeof stellar masses is a prerequisite to understanding evolution and feedback oflow-metallicity massive stars. Using abundant spectroscopic and photometricmeasurements of an outstandingly bright eclipsing binary, we compare itsdynamic, spectroscopic, and evolutionary mass estimates and develop a binaryevolution scenario. We comprehensively studied the eclipsing binary system, AzV476, in the Small Magellanic Cloud. The light curve and radial velocities wereanalyzed to obtain the orbital parameters. The photometric and spectroscopicdata in the UV and optical were analyzed using the Potsdam Wolf-Rayet modelatmospheres. The obtained results are interpreted using binary-evolutiontracks. 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 fromboth the orbital and spectroscopic analysis. The wind mass-loss rate oflog($\dot{M}$/(M$_{\odot}$/yr))=-6.2 of the primary is a factor of ten higherthan a recent empirical prescription for single O stars in the SMC. Onlyclose-binary evolution with mass transfer can reproduce the current stellar andorbital parameters. The binary evolutionary model reveals that the primary haslost about half of its initial mass and is already core helium burning. Ourcomprehensive analysis of AzV 476 yields a consistent set of parameters andsuggests previous case B mass transfer. The derived stellar masses agree withintheir uncertainties. The moderate masses of AzV 476 underline the scarcity ofbright massive stars in the SMC. The core helium burning nature of the primaryindicates that stripped stars might be hidden among OB-type populations.

The earliest O-type eclipsing binary in the Small Magellanic Cloud, AzV 476: A comprehensive analysis reveals surprisingly low stellar masses