Spin-down of the white dwarf in the DQ Herculis system FO Aquarii (H2215-086)

High-speed photometry of FO Aqr (H2215-086) has been obtained in order to update the ephemerides for the orbital and 21 min periodicities. The analysis yields a refined orbital period P O.168018(1) day with no evidence for a period change. The data show that the period of the 21 min optical modulation is increasing as reported by previous authors. This period change is interpreted as spin-down of the white dwarf. By combining the present data with pulse timings from the literature and from a few unpublished sources it is found that P = (8.0 +- 1.1)x 10-11. (author)