Context :

Aims : We report the detection of a magnetic field in the helium-strong star CPD -57 ^ { \circ } 3509 ( B2 IV ) , a member of the Galactic open cluster NGC 3293 , and characterise the star ’ s atmospheric and fundamental parameters .

Methods : 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 .

Results : 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 ^ { \circ } 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 23 750 \pm 250 K and 4.05 \pm 0.10 , respectively , and a surface helium fraction of 0.28 \pm 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 ^ { \circ } 3509 is one of the most evolved He-strong stars known with an independent age constraint due to its cluster membership .

Conclusions :