X-ray long term lightcurve of Wd1-9

Wd1-9 exhibits significant long-term X-ray variability, within which we identify a strong ~14-day periodic signal. We interpret this as the orbital period, marking the first period determination for the system. The X-ray spectrum of Wd1-9 is thermal and hard (kT~3.0keV), resembling the spectra of bright Wolf-Rayet (WR) binaries in Wd1, while a strong Fe emission line at 6.7 keV indicates hot plasma from a colliding-wind X-ray binary.

Wd1-9, with evidence of past mass loss, circumbinary material, a hard X-ray spectrum, and a newly detected 14-day period, displays all the hallmarks of a binary-likely a WR+OB -- that recently underwent early Case B mass transfer. Its sgB[e] classification is likely phenomenological reflecting emission from the dense circumbinary material. This places Wd1-9 in a rarely observed phase, possibly revealing a newly formed WN star, bridging the gap between immediate precursors and later evolutionary stages in Wd1.