  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 minutes. We have performed a new investigation of 21 Com
using MOST satellite and high-cadence ground-based photometry, time series
spectroscopy, and evolutionary and pulsational modeling. 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 Teff =
8900(200) K, log g = 3.9(2), and vsini = 63(2) km/s. Our modeling efforts
suggest that 21 Com is a main sequence (MS) star seen equator-on with a mass of
2.29(10) M(Sun) and a radius of R = 2.6(2) R(Sun). Our extensive photometric
data confirm the existence of rotational light variability with a period of
2.05219(2) d. However, no significant frequencies with a semi-amplitude
exceeding 0.2 mmag were found in the frequency range from 5 to 399 c/d. 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 =
1.41645 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 delta Scuti nor a roAp pulsator but a "well-behaved" CP2 star
exhibiting its trademark rotational variability.
