  Long term X-ray monitoring data from the RXTE ASM and CGRO BATSE reveal that
the third (superorbital) period in SMC X-1 is not constant, but varies between
40-60 days. A dynamic power spectrum analysis indicates that the third period
has been present continuously throughout the five years of ASM observations.
This period changed smoothly from 60 days to 45 days and then returned to its
former value, on a timescale of approximately 1600 days. During the nearly 4
years of overlap between the CGRO & RXTE missions, the simultaneous BATSE hard
X-ray data confirm and extend this variation in SMC X-1. Our discovery of such
an instability in the superorbital period of SMC X-1 is interpreted in the
context of recent theoretical studies of warped, precessing accretion disks. We
find that the behaviour of SMC X-1 is consistent with a radiation-driven
warping model.
