The accelerating rotation of the magnetic He -weak star HD 142990

HD 142.990 (V 913 Sco B5 V) is a Ile-weak star with a strong surface magnetic field and a short rotation period (Pro,- 1 d). Whilst it is clearly a rapid rotator, recent determinations of Prot are in formal disagreement. In this paper, we collect magnetic and photometric data with a combined 40-yr baseline in order to re-evaluate Prot and examine its stability. Both period analysis of individual data sets and 0- C analysis of the photometric data demonstrate that Prot has decreased over the past 30 yr, violating expectations from magnetospheric braking models, but consistent with behaviour reported for 2. other hot, rapidly rotating magnetic stars, CI1 Vir and I-ID 37776. The available magnetic and photometric time series for FID 142.990 can be coherently phased assuming a spin-up rate P of approximately-0.6 s yr-I, although there is some indication that P may have slowed in recent years, possibly indicating an irregular or cyclic rotational evolution.