Wolf-Rayet stars in the Magelanic Clouds. III. The WO4+O4 V binary Sk188 in the SMC.

Spectroscopic observations and radial velocities of the WO4+O4 V binary Sk 188 in the SMC are reported. Emission and antiphased absorption line orbits show that this massive system contains a Wolf-Rayet (W-R) star with relatively low mass ratio of roughly 0.27. It is interpreted to be the most evolved W-R star known in a close, massive binary and fits the scenario of Barlow and Hummer that oxygen-rich WO stars are in a late WC stage as a result of advanced nuclear burning.