On the Stability of the 421-day Periodicity in A0538-66

In this paper we analyse 70 years of archival Harvard and Schmidt plate data of the 16.6 d Be X-ray binary A0538-66 in order to search for the presence of the long-term period of 420.82 +/- 0.79 d found in MACHO photometry (Alcock et al. 2001). We find evidence for a long-term period of 421.29 +/- 0.95 d in the archival data, and examine its stability. We also combine the archival and MACHO datasets in order to improve the accuracy of the orbital period determination, using a cycle-counting analysis to refine its value to 16.6460 +/- 0.0004 d. We also test the model proposed in our previous paper (Alcock et al. 2001) with observations documented in the literature for A0538-66 from 1980-1995, constraining the system inclination to be i > 74.9 +/- 6.5 deg.