Context : \theta Carinae belongs to a group of peculiar early-type stars ( OBN ) with enhanced nitrogen and carbon deficiency .
It is also known as a binary system , but it is not clear yet whether the chemical anomalies can be explained by mass transfer between the two components .
On the basis of the previously reported spectral variability of a few metal lines it may be expected that \theta Car possesses a weak magnetic field .

Aims : A study of the physical nature of this hot massive binary which is furthermore a well-known blue straggler lying \sim 2 mag above the turnoff of the young open cluster IC 2602 is important to understand the origin of its strong chemical anomalies .

Methods : We acquired high resolution spectroscopic and low resolution spectropolarimetric observations to achieve the following goals : a ) to improve the orbital parameters to allow a more in-depth discussion on the possibility of mass transfer in the binary system , b ) to carry out a non-local thermodynamic equilibrium ( NLTE ) abundance analysis , and c ) to search for the presence of a magnetic field .

Results : The study of the radial velocities using CORALIE spectra allowed us to significantly improve the orbital parameters .
A comparative NLTE abundance analysis was undertaken for \theta Car and two other early B-type stars with recently detected magnetic fields , \tau Sco and \xi ^ { 1 } CMa .
The analysis revealed significantly different abundance patterns : a one-order-of-magnitude nitrogen overabundance and carbon depletion was found in \theta Car , while the oxygen abundance is roughly solar .
For the stars \xi ^ { 1 } CMa and \tau Sco the carbon abundance is solar and , while an N excess is also detected , it is of much smaller amplitude ( 0.4–0.6 dex ) .
Such an N overabundance is typical of the values already found for other slowly-rotating ( magnetic ) B-type dwarfs .
For \theta Car , we attribute instead the chemical peculiarities to a past episode of mass transfer between the two binary components .
The results of the search for a magnetic field using FORS 1 at the VLT consisting of 26 measurements over a time span of \sim 1.2 h are rather inconclusive : only few measurements have a significance level of 3 \sigma .
Although we detect a periodicity of the order of \sim 8.8 min in the dataset involving the measurements on all hydrogen Balmer lines with the exception of the H \alpha and H \beta lines , these results have to be confirmed by additional time-resolved magnetic field observations .

Conclusions :