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The magnetic chemically peculiar (Ap/CP2) star 21 Com has been extensivelystudied in the past, albeit with widely differing and sometimes contradictoryresults, in particular concerning the occurrence of short term variabilitybetween about 5 to 90 minutes. We have performed a new investigation of 21 Comusing MOST satellite and high-cadence ground-based photometry, time seriesspectroscopy, and evolutionary and pulsational modeling. Our analysis confirmsthat 21 Com is a classical CP2 star showing increased abundances of, inparticular, Cr and Sr. From spectroscopic analysis, we have derived Teff =8900(200) K, log g = 3.9(2), and vsini = 63(2) km/s. Our modeling effortssuggest that 21 Com is a main sequence (MS) star seen equator-on with a mass of2.29(10) M(Sun) and a radius of R = 2.6(2) R(Sun). Our extensive photometricdata confirm the existence of rotational light variability with a period of2.05219(2) d. However, no significant frequencies with a semi-amplitudeexceeding 0.2 mmag were found in the frequency range from 5 to 399 c/d. Our RVdata also do not indicate short-term variability. We calculated pulsationalmodels assuming different metallicities and ages, which do not predict theoccurrence of unstable modes. The star 18 Com, often employed as comparisonstar for 21 Com in the past, has been identified as a periodic variable (P =1.41645 d). While it is impossible to assess whether 21 Com has exhibitedshort-term variability in the past, the new observational data and severalissues/inconsistencies identified in previous studies strongly suggest that 21Com is neither a delta Scuti nor a roAp pulsator but a "well-behaved" CP2 starexhibiting its trademark rotational variability.

A revisit to the enigmatic variable star 21 Comae