  Using data from the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer
(RXTE) we have searched for long term periodicities in the X-ray flux of GX
1+4, Sco X-2 (GX 349+2), and GX 339-4. For GX 1+4 we also used data from BATSE
and Galactic Centre scans performed by RXTE. We find no evidence for X-ray
modulations at the suggested ~304 d orbital period of GX 1+4. However, we find
tentative evidence for a periodicity at 420 d to 460 d. An upper limit of 15%
peak-to-peak is set on any sinusoidal modulation in the 1.5 - 3.0 keV flux of
Sco X-2 for periods in the 30 to 100 d range. For GX 339-4 we confirm the Low
State modulation and report the detection of significant low-frequency
modulations in both the High State and Very High State. We fail to detect this
modulation in the Off State. We show that if the reported orbital period of GX
339-4 lies in the range 0.5 - 1.7 d, then it is not present in the RXTE ASM
light curve.
