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We report the detection of a new low-mass stellar companion to the whitedwarf HIP 3678 A, the central star of the planetary nebula NGC 246. The newlyfound companion is located about 1 arcsec (at projected separation of about 500au) north-east of HIP 3678 A, and shares a common proper motion with the whitedwarf and its known comoving companion HIP 3678 B. The hypothesis that thenewly detected companion is a non-moving background object can be rejected on asignificance level of more than 8 $\sigma$, by combining astrometricmeasurements from the literature with follow-up astrometry, obtained with WildField Planetary Camera 2/\textit{Hubble Space Telescope} and NACO/Very LargeTelescope. From our deep NACO imaging data, we can rule out additional stellarcompanions of the white dwarf with projected separations between 130 up to 5500au. In the deepest high-contrast NACO observation, we achieve a detection limitin the \textit{Ks} band of about 20 mag, which allows the detection of browndwarf companions with masses down to 36$\,\rmn{M_{jup}}$ at an assumed age ofthe system of 260 Myr. To approximate the masses of the companions HIP 3678 Band C, we use the evolutionary Baraffe et al. models and obtain about0.85\,M$_{\sun}$ for HIP 3678 B and about 0.1 M$_{\sun}$ for HIP 3678 C.According to the derived absolute photometry, HIP 3678 B should be a early tomid-K dwarf (K2--K5), while HIP 3678 C should be a mid-M dwarf with a spectraltype in the range between M5 and M6.

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