  Aims: I seek to fully know the stellar and substellar populations in the
young sigma Orionis open cluster, which is a benchmark for star-forming
studies. Because of the very low proper motion of the cluster, late-type dwarfs
with appreciable proper motion in the foreground of sigma Orionis can be easily
discarded for expensive spectroscopic follow-up. Methods: I use the Aladin sky
atlas, USNO-B1, public astrometric catalogues, and photographic plate
digitisations to identify stars with proper motions inconsistent with cluster
membership in a circular area of radius 30 arcmin centred on the early-type
multiple system sigma Ori. Mostly due to the long time baseline, of more than
half a century, the errors in the measured proper motions are lower than 2
mas/a. Results: Of the 42 stars selected for astrometric follow-up, 37 of them
are proper-motion cluster interlopers. Some USNO-B1 measurements were affected
by partially resolved (visual) multiplicity and target faintness. Because of
their late spectral types and, hence, red colours, 24 contaminants had been
considered as sigma Orionis members at some moment. I discuss how contamination
may have affected previous works (especially related to disc frequencies) and
the curious presence of lithium absorption in three M-dwarf proper-motion
contaminants. Finally, I classify the bright star HD 294297 as a late-F field
dwarf unrelated to the cluster based on a new proper motion measurement.
Conclusions: Although proper motions cannot be used to confirm membership in
sigma Orionis, they can be used instead to discard a number of cluster member
candidates without spectroscopy.
