2025ApJ...985..162L

KS 1947+300, a Be/X-ray binary system, has been observed to experience both type I and type II X-ray outbursts despite its nearly circular orbit, which is atypical according to the viscous decretion disk model. To understand the variations in optical and X-ray, we have analyzed data covering more than 20 yr of observations. In the optical band, we analyzed spectra and light curves, measuring the strength of the Hα and He Iλ6678 lines. In the X-ray band, we studied the long-term light curves provided by several all-sky monitors. KS 1947+300 exhibits changes in brightness and Hα emission on timescales from months to years. The optical and IR variability shows small amplitude changes during the active X-ray state, and a long, smooth decrease during the quiescent state. The fact that the amplitude of variability increased with wavelength suggests that the long-term decrease in the optical emission is due to weakening of the circumstellar disk. Structural changes in the disk may also be the origin of the periodic signals with periods ∼200 days detected in the Zwicky Transient Facility (ZTF)-g and ZTF-r band light curves. These changes could be attributed to the mechanism that ejects matter from the photosphere of the Be star into the disk. In conclusion, the long-term changes in optical continuum and line emission, as well as the X-ray variability patterns, are attributed to an evolving and distorted decretion disk.