The origin of the magnetic fields observed in some intermediate mass and high mass main sequence stars is still a matter of vigorous debate .
The favoured hypothesis is a fossil field origin , in which the observed fields are the condensed remnants of magnetic fields present in the original molecular cloud from which the stars formed .
According to this theory a few percent of the PMS Herbig Ae/Be star should be magnetic with a magnetic topology similar to that of main sequence intermediate-mass stars .

After our recent discovery of four magnetic Herbig stars , we have decided to study in detail one of them , HD 200775 , to determine if its magnetic topology is similar to that of the main sequence magnetic stars .
With this aim , we monitored this star in Stokes I and V over more than two years , using the new spectropolarimeters ESPaDOnS at CFHT , and Narval at TBL .

By analysing the intensity spectrum we find that HD 200775 is a double-lined spectroscopic binary system , whose secondary seems similar , in temperature , to the primary .
We have carefully compared the observed spectrum to a synthetic one , and we found no evidence of abundance anomalies in its spectrum .
We infer the luminosity ratio of the components from the Stokes I profiles .
Then , using the temperature and luminosity of HD 200775 found in the literature , we estimate the age , the mass and the radius of both components from their HR diagram positions .
From our measurements of the radial velocities of both stars we determine the ephemeris and the orbital parameters of the system .

A Stokes V Zeeman signature is clearly visible in most of the Least Square Deconvolution profiles and varies on a timescale on the order of one day .
We have fitted the 30 profiles simultaneously , using a \chi ^ { 2 } minimisation method , with a centered and a decentered-dipole model .
The best-fit model is obtained with a reduced \chi ^ { 2 } = 1.0 and provides a rotation period of 4.3281 \pm 0.0010 Â d , an inclination angle of 60 \pm 11 ^ { \circ } , and a magnetic obliquity angle \beta = 125 \pm 8 ^ { \circ } .
The polar strength of the magnetic dipole field is 1000 \pm 150 Â G , which is decentered by 0.05 \pm 0.04 Â R _ { * } from the center of the star .
The derived magnetic field model is qualitatively identical to those commonly observed in the Ap/Bp stars .

Our determination of the inclination of the rotation axis leads to a radius of the primary which is smaller than that derived from the HR diagram position .
This can be explained by a larger intrinsic luminosity of the secondary relative to the primary , due to a larger circumstellar extinction of the secondary relative to the primary .