  A longitudinal magnetic field with a strength of 5.2kG was recently detected
in CPD -62 2124, which has a fractional main-sequence lifetime of about 60%.
Strongly magnetic early-B type chemically peculiar stars in an advanced
evolutionary state are of special interest to understand the evolution of the
angular momentum and spin-down timescales in the presence of a global magnetic
field. We exploited 17 FORS2 low-resolution spectropolarimetric observations
and 844 ASAS3 photometric measurements for the determination of the rotation
period, pulsations, and the magnetic field geometry of the star. We calculated
periodograms and applied phenomenological models of photometric, spectral and
spectropolarimetric variability. We found that all quantities studied,
specifically equivalent widths, the mean longitudinal magnetic field <B_z>, and
the flux in the V filter, vary with the same period P = 2.628d, which was
identified as the rotation period. The observed variations can be fully
explained by a rigidly rotating main-sequence star with an uneven distribution
of chemical elements, photometric spots, and a stable, nearly dipolar magnetic
field with a polar field strength of about 21kG, frozen into the body of the
star. The magnetic field of CPD -62 2124 is tilted to the rotation axis by
beta=28+-7deg, while the inclination of the rotation axis towards the line of
sight is only i=20+-5deg. In the acquired FORS2 spectra, we detect short-term
line profile variations indicating the presence of beta Cephei type pulsations.
As of today, no other pulsating star of this type is known to possess such a
strong magnetic field.
