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Thanks to long-term X-ray monitoring, a number of interacting binaries arenow known to show X-ray periodicities on timescales of tens to hundreds ofbinary orbits. In some systems, precession of a warped accretion disc is theleading model to explain the superorbital modulation. The High Mass X-rayBinary SMC X-1 showed two excursions in superorbital period (from ~60d to ~45d)during the 1996-2011 interval, suggesting that some characteristic of theaccretion disc is varying on a timescale of years. Because its behaviour as anX-ray pulsar has also been intensively monitored, SMC X-1 offers the rarechance to track changes in both the accretion disk and pulsar behaviours overthe same interval. We have used archival X-ray observations of SMC X-1 toinvestigate whether the evolution of its superorbital variation and pulseperiod are correlated. We use the 16-year dataset afforded by the RXTE~All-SkyMonitor to trace the behaviour of the warped accretion disc in this system, anduse published pulse-period histories to trace the behaviour of the pulsar.While we cannot claim a strong detection of correlation, the first superorbitalperiod excursion near MJD 50,800 does coincide with structure in SMC X-1'spulse period history. Our preferred interpretation is that the superorbitalperiod excursion coincides with a change in the long-term spin-up rate of theSMC X-1 pulsar. In this scenario, the pulsar and the accretion disc are bothresponding to a change in the accretion flow, which the disc itself mayregulate.

Longterm properties of accretion discs in X-ray Binaries - III. A search for spin-superorbital correlation in SMC X-1