  A rather strong mean longitudinal magnetic field of the order of a few
hundred Gauss was detected a few years ago in the Of?p star CPD -28 2561 using
FORS2 low-resolution spectropolarimetric observations. In this work we present
additional low-resolution spectropolarimetric observations obtained during
several weeks in 2013 December using FORS2 (FOcal Reducer low dispersion
Spectrograph) mounted at the 8-m Antu telescope of the VLT. These observations
cover a little less than half of the stellar rotation period of 73.41d
mentioned in the literature. The behaviour of the mean longitudinal magnetic
field is consistent with the assumption of a single-wave variation during the
stellar rotation cycle, indicating a dominant dipolar contribution to the
magnetic field topology. The estimated polar strength of the surface dipole B_d
is larger than 1.15kG. Further, we compared the behaviour of the line profiles
of various elements at different rotation phases associated with different
magnetic field strengths. The strongest contribution of the emission component
is observed at the phases when the magnetic field shows a negative or positive
extremum. The comparison of the spectral behaviour of CPD -28 2561 with that of
another Of?p star, HD148937 of similar spectral type, reveals remarkable
differences in the degree of variability between both stars. Finally, we
present new X-ray observations obtained with the Suzaku X-ray Observatory. We
report that the star is X-ray bright with log L_X/L_bol ~= -5.7. The low
resolution X-ray spectra reveal the presence of a plasma heated up to 24MK. We
associate the 24MK plasma in CPD -28 2561 with the presence of a kG strong
magnetic field capable to confine stellar wind.
