  Magnetic early B-type stars are rare. To identify them, we use the strongest
indirect indicator of a magnetic field in B stars, which is periodic
variability of UV stellar wind lines occurring symmetric about the approximate
rest wavelength. Probable magnetic candidates are targets for follow-up
spectropolarimetry to search for a magnetic field. From the UV wind line
variability the B1/B2V star sigma Lupi emerged as a new magnetic candidate
star. AAT spectropolarimetric measurements with SEMPOL were obtained. The
longitudinal component of the magnetic field integrated over the visible
surface of the star was determined with the Least-Squares Deconvolution method.
The UV line variations of sigma Lupi are similar to what is known in magnetic B
stars, but no periodicity could be determined. We detected a varying
longitudinal magnetic field with amplitude of about 100 G with error bars of
typically 20 G, which supports an oblique magnetic-rotator configuration. The
EW variations of the UV lines, the magnetic and the optical-line variations are
consistent with the photometric period of 3.02 d, which we identify with the
rotation period of the star. Additional observations with ESPaDOnS attached to
the CFHT confirmed this discovery, and allowed the determination of a precise
magnetic period. Analysis revealed that sigma Lupi is a He-strong star, with an
enhanced N abundance and an underabundance of C, and has a chemically spotted
surface. Like in other magnetic B stars the UV wind emission appears to
originate close to the magnetic equatorial plane, with maximum emission when a
magnetic pole points towards the Earth. The 3.01972(43) d magnetic rotation
period is consistent with the photometric period, with maximum light
corresponding to maximum magnetic field.
