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We report magnetic and spectroscopic observations and modeling of the Of?pstar CPD -28 2561. Using more than 75 new spectra, we have measured theequivalent width variations and examined the dynamic spectra of photosphericand wind-sensitive spectral lines. A period search results in an unambiguous73.41 d variability period. High resolution spectropolarimetric data analyzedusing Least-Squares Deconvolution yield a Zeeman signature detected in the meanStokes V profile corresponding to phase 0.5 of the spectral ephemeris.Interpreting the 73.41 d period as the stellar rotational period, we havephased the equivalent widths and inferred longitudinal field measurements. Thephased magnetic data exhibit a weak sinusoidal variation, with maximum of about565 G at phase 0.5, and a minimum of about -335 G at phase 0.0, with extremaapproximately in phase with the (double-wave) Halpha equivalent widthvariation. Modeling of the Halpha equivalent width variation assuming aquasi-3D magnetospheric model produces a unique solution for the ambiguouscouplet of inclination and magnetic obliquity angles: (i, beta) or (beta,i)=(35 deg,90 deg). Adopting either geometry, the longitudinal field variationyields a dipole polar intensity Bd=2.6\pm 0.9~kG, consistent with that obtainedfrom direct modelling of the Stokes V profiles. We derive a wind magneticconfinement parameter eta*\simeq 100, leading to an Alfv\'en radius RA\simeq3-5~R*, and a Kepler radius RK\simeq 20~R*. This supports a physical scenarioin which the Halpha emission and other line variability have their origin in anoblique, co-rotating 'dynamical magnetosphere' structure resulting from amagnetically channeled wind. Nevertheless, the details of the formation ofspectral lines and their variability within this framework remain generallypoorly understood.

Rotation, spectral variability, magnetic geometry and magnetosphere of the Of?p star CPD -28 2561