A search is made for periodic modulation in the X-ray flux from the low mass X-ray binary GX13+1 using Rossi X-ray Timing Explorer All Sky Monitor data collected over a period of almost seven years .
From a filtered data set , which excludes measurements with exceptionally large error bars and so maximizes signal to noise , modulation is found at a period of 24.065 \pm 0.018 days .
The modulation is most clearly detectable at high energies ( 5 - 12 keV ) .
Spectral changes are revealed as a modulation in hardness ratio on the 24 day period and there is a phase shift between the modulation in the 5 - 12 keV energy band compared to the 1.5 - 5 keV band .
The high-energy spectrum of GX13+1 is unusual in displaying both emission and absorption iron line features and it is speculated that the peculiar spectral and timing properties may be connected .