The demographics of binary companions to stripped-envelope supernovae: confronting population synthesis models with observations

ABSTRACT Stripped-envelope supernovae (SESNe) mark the deaths of massive stars without hydrogen-rich envelopes. Most SESNe likely originate from binary systems where a companion stripped the progenitor of its envelope. Years of HST imaging of nearby SESN sites have produced a statistically meaningful sample of constraints on surviving binary companions. We assemble the current sample of six companion detections and six non-detections from the literature, re-analysing whenever needed. We then conduct the first statistical comparison with binary population-synthesis predictions, primarily based on new calculations performed with the POSYDON framework. Across a metallicity range, our models predict that 80–90 % of Type Ib/c and 60–85 % of IIb SNe explode with a rapidly rotating main-sequence companion. The observed luminosity distribution favours fairly inefficient mass accretion and failed explosions of the most massive stripped stars. The companion detection fraction broadly matches predictions, given the imaging depth, but appears elevated for SN IIb. In all but one non-detection, a faint undetected companion is the most likely scenario. The red apparently evolved companions in a few SN Ib/c may result from strong interaction with the ejecta, expected in $\sim 12~\%$ of them. Companion demographics offer a powerful independent probe of SESN progenitor systems, with the current sample disfavouring efficient accretion and supporting Wolf–Rayet non-explodability. Larger companion samples and follow-up studies will further clarify binary pathways to SESNe, serving as benchmarks for transient surveys.