2008A&A...487..323S

The young O-type star θ¹OriC, the brightest star of the Trapezium cluster in Orion, is one of only two known magnetic rotators among the O stars. However, not all spectroscopic variations of this star can be explained by the magnetic rotator model. We present results from a long-term monitoring to study these unexplained variations and to improve the stellar rotational period. We want to study long-term trends of the radial velocity of θ¹OriC, to search for unusual changes, to improve the established rotational period and to check for possible period changes. We combine a large set of published spectroscopic data with new observations and analyze the spectra in a homogeneous way. We study the radial velocity from selected photo-spheric lines and determine the equivalent width of the Hα and HeIIλ4686 lines. We find evidence for a secular change of the radial velocity of θ¹OriC that is consistent with the published interferometric orbit. We refine the rotational period of θ¹OriC and discuss the possibility of detecting period changes in the near future.