  A 24 day period for the low-mass X-ray binary GX 13+1 was previously proposed
on the basis of 7 years of RXTE ASM observations (Corbet 2003) and it was
suggested that this was the orbital period of the system. This would make it
the one of the longest known orbital periods for a Galactic low-mass X-ray
binary powered by Roche lobe overflow. We present here the results of: (i)
K-band photometry obtained with the SMARTS Consortium CTIO 1.3 telescope on 68
nights over a 10 month interval; (ii) Continued monitoring with the RXTE ASM,
analyzed using a semi-weighted power spectrum instead of the data filtering
technique previously used; and (iii) Swift BAT hard X-ray observations.
Modulation near 24 days is seen in both the K-band and additional statistically
independent ASM X-ray observations. However, the modulation in the ASM is not
strictly periodic. The periodicity is also not detected in the Swift BAT
observations, but modulation at the same relative level as seen with the ASM
cannot be ruled out. If the 24 day period is the orbital period of system, this
implies that the X-ray modulation is caused by structure that is not fixed in
location. A possible mechanism for the X-ray modulation is the dipping behavior
recently reported from XMM-Newton observations.
