Probable nature of HD 196821: Is the star a cool HgMn?

The abundance and detailed chemical analysis of HD 196821 are discussed, based on medium- and high-resolution spectra. The nature of this star is controversial: it has been classified variously as lambda$$ \lambda $$ Bootis, Ap of the Si-Cr group, and HgMn. The atmospheric parameters are determined from the H-beta profile line from the medium-resolution spectra. We measure the chemical abundances of 19 atomic species from the high-resolution spectrum. The atmosphere of this star has slightly lower abundances of the light elements oxygen and magnesium with respect to solar values. Nevertheless, phosphorus, scandium, and chromium, as well as the heavy elements Sr, Y, Ba, and Yb, are enhanced compared to the solar values. The abundances of silicon, titanium, iron, and nickel are close to solar. HD 196821 shows an obvious overabundance of Mn II (2.11 dex) and Hg II (5.32 dex). Such a pattern is characteristic of chemically peculiar stars, particularly mercury-manganese stars. We also investigate the possible photometric variability of the star. The transiting exoplanet survey satellite light curve of HD 196821 reveals a rotational period of 5.7182 days and a harmonic of 2.8286 days, indicating the presence of high-contrast star-spots on its surface. According to these indicators, we conclude that the star is a mercury-manganese star.