We wish to confirm the nature of YSES 2b, a purportedly faint companion of the young star YSES 2. We used on-sky observations from SPHERE and GRAVITY to measure the astrometric position of 2b with respect to the star YSES 2, and examined the competing hypotheses of (i) a bound substellar companion versus (ii) a distant unrelated background source with a non-zero proper motion. YSES 2b appears to be a late-type M-dwarf star over 2 kiloparsecs behind the star YSES 2. It has a transverse velocity of about 300 km/s and is located within one of the spiral arms of the Galaxy. The main discriminant was multiple epochs of GRAVITY astrometry that identified the sub-milliarcsecond parallactic motion of the star.
@article{arxiv.2509.09504,
title = {YSES 2b is a background star: Differential astrometric M-dwarf measurements in time},
author = {Matthew Kenworthy and Tomas Stolker and Jens Kammerer and William Balmer and Arthur Vigan and Sylvestre Lacour and Gilles Otten and Eric Mamajek and Christian Ginski and Mathias Nowak and Steven Martos and Jason Wang and Emily Rickman and Markus Janson and Alexander Bohn and Mariangela Bonavita},
journal= {arXiv preprint arXiv:2509.09504},
year = {2025}
}