HIP 3678: a hierarchical triple stellar system in the centre of the planetary nebula NGC 246

We report the detection of a new low-mass stellar companion to the white dwarf HIP 3678 A, the central star of the planetary nebula NGC 246. The newly found companion is located about 1 arcsec (at projected separation of about 500 au) north-east of HIP 3678 A, and shares a common proper motion with the white dwarf and its known comoving companion HIP 3678 B. The hypothesis that the newly detected companion is a non-moving background object can be rejected on a significance level of more than 8σ, by combining astrometric measurements from the literature with follow-up astrometry, obtained with Wide Field Planetary Camera 2/Hubble Space Telescope and NACO/Very Large Telescope. From our deep NACO imaging data, we can rule out additional stellar companions of the white dwarf with projected separations between 130 up to 5500 au. In the deepest high-contrast NACO observation, we achieve a detection limit in the Ks band of about 20 mag, which allows the detection of brown dwarf companions with masses down to 36 M_jup at an assumed age of the system of 260 Myr. To approximate the masses of the companions HIP 3678 B and C, we use the evolutionary Baraffe et al. models and obtain about 0.85 M_⊙ for HIP 3678 B and about 0.1 M_⊙ for HIP 3678 C. According to the derived absolute photometry, HIP 3678 B should be a early to mid-K dwarf (K2-K5), while HIP 3678 C should be a mid M dwarf with a spectral type in the range between M5 and M6.