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(abridged version) Context : The detection of bright, hard, and variableX-ray emission in Tr16-22 prompted spectropolarimetric observations of thisstar, which in turn led to the discovery of a surface magnetic field. Aims : Wewant to further constrain the properties of this star, in particular to verifywhether X-ray variations are correlated to changes in optical emission linesand magnetic field strength, as expected from the oblique rotator model that iswidely accepted for magnetic O stars. Methods: We have obtained newlow-resolution spectropolarimetric and long-term high-resolution spectroscopicmonitoring of Tr16-22, and we also analyse new, serendipitous X-ray data.Results: The new X-ray observations are consistent with previous data, buttheir addition does not help to solve the ambiguity in the variation timescalebecause of numerous aliases. No obvious periodicity or any large variations aredetected in the spectropolarimetric data of Tr16-22 obtained over three months.The derived field values appear to be in line with previous measurements,suggesting constancy of the field (though the possibility of small, short-termfield variations cannot be excluded). Variations in the equivalent widths ofHalpha are very small, and they do not appear to be related to the X-raytimescale; the overall lack of large variations in optical emission lines isconsistent with the magnetic field constancy. In addition, variations of theradial velocities indicate that Tr16-22 is probably a SB1 binary with a verylong period.

The puzzling properties of the magnetic O star Tr16-22