Starspots on active binaries are usually tracked through brightness variations and spectral Doppler imaging. These methods suggest slowly evolving surfaces and constrain differential rotation. But they produce ambiguous spot locations — photometry can’t distinguish between spots at different latitudes, and Doppler imaging has resolution limits.

This paper uses the CHARA Array’s 330-meter baseline to directly image the surface of ζ Andromedae across three epochs spanning six rotations. The interferometric images reveal more complex activity than photometry or earlier Doppler maps suggested. Spot structures change on rotational timescales, faster than the gradual evolution photometric monitoring implied. A polar spot grows over the observing period.

The surprise is the companion. ζ Andromedae is a binary, and the secondary was predicted to be a 0.75 solar-mass main-sequence star. It wasn’t detected despite sufficient sensitivity. The authors suggest it may be a white dwarf instead — compact enough to be invisible but massive enough to explain the orbital dynamics. The imaging that was designed to study starspots may have incidentally reclassified the companion. The surface turned out to be more dynamic than expected, and the companion less luminous.