  The young O-type star theta1 OriC, 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 theta1 OriC, 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 Halpha and HeII4686 lines. We find evidence for a
secular change of the radial velocity of theta1 OriC that is consistent with
the published interferometric orbit. We refine the rotational period of theta1
OriC and discuss the possibility of detecting period changes in the near
future.
