  We report the discovery using data from the Swift Burst Alert Telescope (BAT)
of superorbital modulation in the wind-accretion supergiant high-mass X-ray
binaries 4U 1909+07 (= X 1908+075), IGR J16418-4532, and IGR J16479-4514.
Together with already known superorbital periodicities in 2S 0114+650 and IGR
J16493-4348, the systems exhibit a monotonic relationship between superorbital
and orbital periods. These systems include both supergiant fast X-ray
transients (SFXTs) and classical supergiant systems, and have a range of
inclination angles. This suggests an underlying physical mechanism which is
connected to the orbital period. In addition to these sources with clear
detections of superorbital periods, IGR J16393-4643 (= AX J16390.4-4642) is
identified as a system that may have superorbital modulation due to the
coincidence of low-amplitude peaks in power spectra derived from BAT, RXTE PCA,
and INTEGRAL light curves. 1E 1145.1-6141 may also be worthy of further
attention due to the amount of low-frequency modulation of its light curve.
However, we find that the presence of superorbital modulation is not a
universal feature of wind-accretion supergiant X-ray binaries.
