Exploring the radial velocity variations of RY Lup with VLT/ESPRESSO: Binary versus spot hypotheses

Stellar multiplicity could plausibly lead to the creation of protoplanetary disc substructures, as tidal forces from a close-in spectroscopic stellar companion have the capacity to carve out gaps and shape disc architecture. However, in young, active systems, radial velocity (RV) signatures are often complicated by stellar activity. We investigate RY Lup, a classical T Tauri star hosting a disc with a ~60 au cavity, where studies with Gaia astrometry and VLT/SPHERE imaging have hinted at an unseen companion. Using high-resolution VLT/ESPRESSO spectra and the least-squares deconvolution (LSD) technique, we analysed RV variations over 327 days. We detected significant line profile variations with a periodic signal of ~3.75 days, aligning with prior photometric estimates. The variations are compatible with a close-in binary system at ~0.04 au and a mass ratio of q ~ 0.6. A combined analysis of RV data and ALMA dynamical mass estimates, using 13 CO and 18 CO, revealed a highly misaligned system. The nearly face-on binary i ~ 13° is misaligned to both the inner and outer discs i ~ 50° and ~70°, respectively. The derived orbital separation is compatible with the inner disc size, given an inner rim at a = 0.12 au measured by VLTI/GRAVITY, which suggests a highly warped disc structure. Nonetheless, the short orbital period conflicts with the derived eccentricity ( e ~ 0.23). To explore alternative explanations, we assessed the impact of stellar spots on RV signals. While the LSD deformations can be modelled by different cool spot configurations, a 15% dispersion in retrieved v sin i values – coupled with the lack of a significant periodic signal – suggests that spots alone cannot explain the observed variability. As neither hypothesis has been ruled out, we recommend future combined RV and interferometric monitoring to clarify the nature of spectroscopic variability.