2019MNRAS.485.4247P

The magnetic chemically peculiar (Ap/CP2) star 21 Com has been extensively studied in the past, albeit with widely differing and sometimes contradictory results, in particular concerning the occurrence of short-term variability between about 5 to 90 min. We have performed a new investigation of 21 Com using Microvariability and Oscillations of Star (MOST) satellite and high-cadence ground-based photometry, time series spectroscopy, and evolutionary and pulsational modelling. Our analysis confirms that 21 Com is a classical CP2 star showing increased abundances of, in particular, Cr and Sr. From spectroscopic analysis, we have derived T_ eff_ = 8 900±200 K, log g = 3.9 ± 0.2, and υsin i = 63 ± 2 km s^–1. Our modelling efforts suggest that 21 Com is a main sequence (MS) star seen equator-on with a mass of 2.29 ± 0.10 M_☉ and a radius of R = 2.6 ± 0.2 R_☉. Our extensive photometric data confirm the existence of rotational light variability with a period of 2.052 19(2) d. However, no significant frequencies with a semi-amplitude exceeding 0.2 mmag were found in the frequency range from 5 to 399 d^–1. Our RV data also do not indicate short-term variability. We calculated pulsational models assuming different metallicities and ages, which do not predict the occurrence of unstable modes. The star 18 Com, often employed as comparison star for 21 Com in the past, has been identified as a periodic variable (P = 1.416 45 d). While it is impossible to assess whether 21 Com has exhibited short-term variability in the past, the new observational data and several issues/inconsistencies identified in previous studies strongly suggest that 21 Com is neither a δ Scuti nor a roAp pulsator but a 'well-behaved' CP2 star exhibiting its trademark rotational variability.