Magnetism and binarity of the Herbig Ae star V380 Ori

In this paper we report the results of high-resolution circular spectropolarimetric monitoring of the Herbig Ae star V380 Ori, in which we discovered a magnetic field in 2005. A careful study of the intensity spectrum reveals the presence of a cool spectroscopic companion. By modelling the binary spectrum, we infer the effective temperature of both stars: 10 500 +/- 500 K for the primary and 5500 +/- 500 K for the secondary, and we argue that the high metallicity ([M/H] = 0.5), required to fit the lines, may imply that the primary is a chemically peculiar star. We observe that the radial velocity of the secondary's lines varies with time, while that of the primary does not. By fitting these variations, we derive the orbital parameters of the system. We find an orbital period of 104 +/- 5 d and a mass ratio (M(P)/M(S)) larger than 2.9. The intensity spectrum is heavily contaminated with strong, broad and variable emission. A simple analysis of these lines reveals that a disc might surround the binary and that a wind occurs in the environment of the system. Finally, we performed a magnetic analysis using the least-squares deconvolved profiles of the Stokes V spectra of both stars and adopting the oblique rotator model. From rotational modulation of the primary's Stokes V signatures, we infer its rotation period P = 4.312 76 +/- 0.000 42 d and find that it hosts a centred dipole magnetic field of polar strength 2.12 +/- 0.15 kG, with a magnetic obliquity beta = 66 degrees +/- 5 degrees and a rotation axis inclination i = 32 degrees +/- 5 degrees. However, no magnetic field is detected in the secondary, and if it hosts a dipolar magnetic field, its strength must be below about 500 G, to be consistent with our observations.