  From radial velocities determined in high signal-to-noise digital spectra, we
report the discovery that the brightest component of the binary system HD
150136 is of spectral type O3. We also present the first double-lined orbital
solution for this binary. Our radial velocities confirm the previously
published spectroscopic orbital period of 2.6 days. He II absorptions appear
double at quadratures, but single lines of N V and N IV visible in our spectra
define a radial velocity orbit of higher semi-amplitude for the primary
component than do the He II lines. From our orbital analysis, we obtain minimum
masses for the binary components of 27 and 18$\modot$. The neutral He
absorptions apparently do not follow the orbital motion of any of the binary
components, thus they most probably arise in a third star in the system.
