Solar-like oscillations and magnetic activity of the slow rotator EK Eridani

International audience Aims: We aim to understand the interplay between non-radial oscillations and stellar magnetic activity and test the feasibility of doing asteroseismology of magnetically active stars. We investigate the active slow rotator EK Eri which is the likely descendant of an Ap star. Methods: We analyze 30 years of photometric time-series data, 3 years of HARPS radial velocity monitoring, and 3 nights of high-cadence HARPS asteroseismic data. We construct a high-S/N HARPS spectrum that we use to determine atmospheric parameters and chemical composition. Spectra observed at different rotation phases are analyzed to search for signs of temperature or abundance variations. An upper limit on the projected rotational velocity is derived from very high-resolution CES spectra. Results: We detect oscillations in EK Eri with a frequency of the maximum power of nu_max = 320 ± 32 muHz, and we derive a peak amplitude per radial mode of ≈0.15 m s-1, which is a factor of ≈3 lower than expected. We suggest that the magnetic field may act to suppress low-degree modes. Individual frequencies can not be extracted from the available data. We derive accurate atmospheric parameters, refining our previous analysis, finding T_eff = 5135 ± 80 K, log g = 3.39 ± 0.12, and metallicity [M/H] = + 0.02 ± 0.04. Mass and radius estimates from the seismic analysis are not accurate enough to constrain the position in the HR diagram and the evolutionary state. We confirm that the main light variation is due to cool spots, but that other contributions may need to be taken into account. We tentatively suggest that the rotation period is twice the photometric period, i.e., P_rot = 2P_phot = 617.6 d, and that the star is a dipole-dominated oblique rotator viewed close to equator-on. We conclude from our derived parameters that v sin i -1 and we show that the value is too low to be reliably measured. We also link the time series of direct magnetic field measurements available in the literature to our newly derived photometric ephemeris