The magnetic field of CPD -57 ^ { \circ } 3509 was recently detected in the framework of the BOB ( B fields in OB stars ) collaboration .
We acquired low-resolution spectropolarimetric observations of CPD -57 ^ { \circ } 3509 with FORS 2 and high-resolution UVES observations randomly distributed over a few months to search for periodicity , to study the magnetic field geometry , and to determine the surface distribution of silicon and helium .
We also obtained supplementary photometric observations at a timeline similar to the spectroscopic and spectropolarimetric observations .
A period of 6.36 d was detected in the measurements of the mean longitudinal magnetic field .
A sinusoidal fit to our measurements allowed us to constrain the magnetic field geometry and estimate the dipole strength in the range of 3.9–4.5 kG .
Our application of the Doppler imaging technique revealed the presence of He i spots located around the magnetic poles , with a strong concentration at the positive pole and a weaker one around the negative pole .
In contrast , high concentration Si iii spots are located close to the magnetic equator .
Further , our analysis of the spectral variability of CPD -57 ^ { \circ } 3509 on short time scales indicates distinct changes in shape and position of line profiles possibly caused by the presence of \beta Cep-like pulsations .
A small periodic variability in line with the changes of the magnetic field strength is clearly seen in the photometric data .