HD 213258: A new rapidly oscillating, super slowly rotating, strongly magnetic Ap star in a spectroscopic binary

We report on HD 213258, an Ap star that we recently identified as presenting a unique combination of rare, remarkable properties. Our study of this star is based on ESPaDOnS Stokes I and V data obtained at seven epochs spanning a time interval slightly shorter than 2 yr, on TESS data, and on radial velocity measurements from the CORAVEL data base. We confirm that HD 213258, which was originally suspected to be an F str lambda 4077 star, is definitely an Ap star. We found that, in its spectrum, the Fe II lambda 6149.2 angstrom line is resolved into its two magnetically split components. The mean magnetic field modulus of HD 213258, ⟨B⟩similar to 3.8 kG, which we determined from this splitting, does not show significant variations over similar to 2 yr. Comparing our mean longitudinal field determinations with a couple of measurements from the literature, we show that the stellar rotation period is likely of the order of 50 yr, with a reversal of the field polarity. Moreover, HD 213258 is a rapidly oscillating Ap (roAp) star, in which high overtone pulsations with a period of 7.58 min are detected. Finally, we confirm that HD 213258 has a mean radial velocity exceeding (in absolute value) that of at least 99% of Ap stars. The radial velocity shows low amplitude variations, which suggests that the star is a single-line spectroscopic binary. It is also a known astrometric binary. While its orbital elements remain to be determined, its orbital period is likely one of the shortest known for a binary roAp star. Its secondary is close to the borderline between stellar and substellar objects. There is a significant probability that it is a brown dwarf. The pair represents a combination that has never been observed before. Most of the above-mentioned properties taken in isolation are only observed in a small fraction of the whole population of Ap stars. Thus, the probability that a single star possesses all of them is extremely low. This makes HD 213258 an exceptionally interesting object that deserves to be studied in detail in the future.