HD 148937: A multiwavelength study of the third galactic member of the of?p class

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, 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).