  We report the detection of a magnetic field in the helium-strong star CPD-57
3509 (B2 IV), a member of the Galactic open cluster NGC3293, and characterise
the star's atmospheric and fundamental parameters. Spectropolarimetric
observations with FORS2 and HARPSpol are analysed using two independent
approaches to quantify the magnetic field strength. A high-S/N FLAMES/GIRAFFE
spectrum is analysed using a hybrid non-LTE model atmosphere technique.
Comparison with stellar evolution models constrains the fundamental parameters
of the star. We obtain a firm detection of a surface averaged longitudinal
magnetic field with a maximum amplitude of about 1 kG. Assuming a dipolar
configuration of the magnetic field, this implies a dipolar field strength
larger than 3.3 kG. Moreover, the large amplitude and fast variation (within
about 1 day) of the longitudinal magnetic field implies that CPD-57 3509 is
spinning very fast despite its apparently slow projected rotational velocity.
The star should be able to support a centrifugal magnetosphere, yet the
spectrum shows no sign of magnetically confined material; in particular,
emission in H{\alpha} is not observed. Apparently, the wind is either not
strong enough for enough material to accumulate in the magnetosphere to become
observable or, alternatively, some leakage process leads to loss of material
from the magnetosphere. The quantitative spectroscopic analysis of the star
yields an effective temperature and a logarithmic surface gravity of 23750+-250
K and 4.05+-0.10, respectively, and a surface helium fraction of 0.28+-0.02 by
number. The surface abundances of C, N, O, Ne, S, and Ar are compatible with
the cosmic abundance standard, whereas Mg, Al, Si, and Fe are depleted by about
a factor of 2. This abundance pattern can be understood as the consequence of a
fractionated stellar wind. CPD-57 3509 is one of the most evolved He-strong
stars known.
