Possible 38 day X - ray period of KS 1731 - 260

We report the detection of a 38 day period in the X-ray flux of the transient burster KS 1731-260 . The narrow peak of periodicity was detected during ∼TJD 10150–11050 when the source had a high and relatively stable X-ray flux. After ∼TJD 11100 the source became strongly variable on a time scale of months that contaminates the search for the 38 day periodicity. The detected period can not be a binary period. The binary with Roche lobe overflow has in this case large radii of the secondary and of the accretion disk. Disk and secondary star illumination by X-ray flux from luminous neutron star would lead to high infrared brightness of the binary. That clearly contradicts the infrared data even for the brightest infrared sources within CHANDRA error box of KS 1731-260 . Remaining possibility is that observed periodicity is connected with the accretion disk precession, similar to that was observed for SS 433, Her X-1, Cyg X-1 etc.