Complex Periodic Variables: star-planet-disc interaction at the substellar frontier

Complex Periodic Variable (CPV) stars, are a fascinating new class of young, fast rotating very low-mass stars recently identified with Kepler and TESS (Bouma et al. 2024), which shows transit-like dips at the co-rotation radius. Although the physical mechanism of these dips is currently unknown, they could be related to the presence of dust material in a debris disk or evaporated from a planet, or to the ejection of coronal gas material from the stars.

Here we summarize the importance of CPVs, some of which are very low mass young binaries close to the substellar borderline. Their dynamical mass determination and understanding of the transiting material can shed light of the brown dwarf frontier at these young ages, planetary formation and star-planet-disc interaction at very low masses. We present the latest results of a photometric, spectroscopic, radial velocity and astrometric monitoring campaign spanning several years in the optical and radio of 2M0508-21, a young, fast rotating (6.7h) M5 binary system belonging to b Pic (~20 Myr), and DG CVn, another young (~150 Myr) and fast rotating (6.44h) M4 binary, that it is the closest known CPV. These studies allowed us to perform a comprehensive multi-band photometric and spectroscopic characterization of the dips and determine the orbit of their binary systems, representing a unique opportunity to shed light on this phenomenon.