A possible 38 day X-ray period of KS 1731-260

We report the detection of a 38 day period of the X-ray transient and X-ray burster KS 1731-260. The periodicity was detected during Mar.1996-Aug.1998 (~TJD 10150-11050) when the source had a high and relatively stable X-ray flux. After Sep.-Oct. 1998 (~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. A binary with Roche lobe overflow has in this case large radii of the secondary and of the accretion disk. Illumination of the disk and secondary star by the X-ray flux from the luminous neutron star and inner disk, would lead to a 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. A remaining possibility is that observed periodicity is connected with the accretion disk precession, similar to that was observed for, e.g. SS 433 and Her X-1.