Abstract

We report an analysis of the published times of orbital minima of FO Aqr and those derived from two new long observations, confirming our earlier report (Osborne & Mukai 1988) that the orbital period of this system is 4.85 hr, the 1-day alias of the widely accepted 4.03 hr period. We also reconsider the white dwarf spin ephemeris using 171 individual pulse times spanning 7 yr. Th average period derivative dottedP over this interval is found to be + (3.4±0.4) × 10^-11. We also find that the period derivative is changing with umlautP = -(2.5±0.8) × 10^-11 yr^-1. The individual pulse times show an orbital trend, which we interpret as the effect of the sideband pulsation; this enables us to locate the reprocessing site to be at inferior conjunction at orbital phase ∼0.8. Inferior conjunction of the secondary star occurs at phase zero, so we suggest that the accretion stream-disc interacfion region is the reprocessing site. Finally, we consider the spin period behaviour; the umlautP term implies either a change in the accretion rate or the presence of a third body.