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Three Galactic O-type stars belong to the rare class of Of?p objects: HD108,HD191612, and HD148937. The first two stars show a wealth of phenomena,including magnetic fields and strong X-ray emission, light variability, anddramatic periodic spectral variability. We present here the first detailedoptical and X-ray study of the third Galactic Of?p star, HD148937.Spectroscopic monitoring has revealed low-level variability in the Balmer andHeII4686 lines, but constancy at HeI and CIII4650. The Ha line exhibits profilevariations at a possible periodicity of ~7d. Model atmosphere fits yieldT_{eff}=41000+-2000K, log(g)=4.0+-0.1, Mdot_{sph}<~ 10^{-7}Msol/yr and asurabondance of nitrogen by a factor of four. At X-ray wavelengths, HD148937resembles HD108 and HD191612 in having a thermal spectrum dominated by arelatively cool component (kT=0.2keV), broad lines (>1700km/s), and anorder-of-magnitude overluminosity compared to normal O stars (log[L_X^unabs/L_BOL] ~ -6).

HD148937: a multiwavelength study of the third Galactic member of the Of?p class