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Three Galactic O-type stars belong to the rare class of Of?p objects: HD 108, HD 191612, and HD 148937. The first two stars show a wealth of phenomena, including strong X-ray emission, light variability, and dramatic periodic spectral variability. We present here the first detailed optical and X-ray study of the third Galactic Of?p star, HD 148937. Spectroscopic monitoring has revealed low-level variability in the Balmer and He ii λ4686 lines, but constancy at He i and C iii λ4650. The Hα line exhibits profile variations at a possible periodicity of ∼7 days. Model atmosphere fits yield T eff = 41000 ± 2000 K, log(g) = 4.0 ± 0.1,Ṁ sph 10 −7 M yr −1 , and an overabundance of nitrogen by a factor of 4. At X-ray wavelengths, HD 148937 resembles HD 108 and HD 191612 in having a thermal spectrum dominated by a relatively cool component (kT = 0.2 keV), broad lines ( 1700 km s −1 ), and an order-of-magnitude overluminosity compared to normal O stars log L unabs X L BOL ∼−6 .

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Eclipsing systems of massive stars allow one to explore the properties of their components in great detail. We perform a multi-wavelength, non-LTE analysis of the three components of the massive multiple system δ Ori A, focusing on the fundamental stellar properties, stellar winds, and X-ray characteristics of the system.The primary's distance-independent parameters turn out to be characteristic for its spectral type (O9.5 II), but usage of the Hipparcos parallax yields surprisingly low values for the mass, radius, and luminosity. Consistent values follow only if δ Ori lies at about twice the Hipparcos distance, in the vicinity of the σ-Orionis cluster. The primary and tertiary dominate the spectrum and leave the secondary only marginally detectable. We estimate the V-band magnitude difference between primary and secondary to be ∆V ≈ 2. m 8. The inferred parameters suggest the secondary is an early B-type dwarf (≈ B1 V), while the tertiary is an early B-type subgiant (≈ B0 IV). We find evidence for rapid turbulent velocities (∼ 200 km s −1 ) and wind inhomogeneities, partially optically thick, in the primary's wind. The bulk of the X-ray emission likely emerges from the primary's stellar wind (log L X /L Bol ≈ −6.85), initiating close to the stellar surface at R 0 ∼ 1.1 R * . Accounting for clumping, the mass-loss rate of the primary is found to be logṀ ≈ −6.4 [M ⊙ yr −1 ] , which agrees with hydrodynamic predictions, and provides a consistent picture along the X-ray, UV, optical and radio spectral domains.