in the period determination. The actual timing behavior is somewhat surprising as there appear to be two spin-up events on similar timescales, one before the gap in coverage and one after. Observations 4 and 10 seem not to fit this picture, which may be attributable to systematic errors resulting from pulse profile variation.

A problem with the spin-up interpretation is the size of the pulse period changes. Standard accretion torque theory [@GL1979] predicts spin period changes of approximately 2.4 $\times$10$^{-11}$ s s$^{-1}$ L$_{38}^{6/7}\mu_{30}^{2/7}$ which is exceeded by factors of up to $\sim$100 (Table 2). Another contribution to period changes can come from orbital motion. For a circular orbit, and assuming a neutron star mass of 1.4 M$_{\sun}$ and a mass donor mass of 10 M$_{\sun}$ then the orbital velocity semi-amplitude is $\sim$170 (15/P$_{orb}$)$^{1/3}$ sin i km s$^{-1}$. where P$_{orb}$ is in units of days. The observed period changes are of at least of this approximate magnitude although an obvious systematic trend was not detected. Finally, if our period determinations are dependent on pulse profile, although we do not expect this, then an artificial variation of pulse period with luminosity may be produced.

The pulse period discovered for SMC X-2 is 