CBA Center for Backyard Astrophysics

FO Aqarii: King of the DQ Her
Stars (A World of Pulse Timings)

FO Aqr figure

One of our jobs in life is to keep watch over the clocks of DQ Her stars. FO Aquarii has the biggest pulses, and at V=13.7 is just about the brightest of this class. So we've been plugging away since 1981. The good recent coverage yields a good cycle count throughout 1981 - 1997, and here are the O - C diagrams. The upper frame shows a linear fit to the times of orbital dips, yielding an ephemeris

Orbital minimum = HJD 2444782.869(2) + 0.2020596(1) E.

Simple enough. The lower frame shows a cubic fit to the times of 21 min (spin) maxima. The rapid plummet in the O - C shows that the period is now decreasing quite rapidly, since P is basically the instantaneous slope of the O - C. The average rate of decrease is dP/dt = 4x10-11 s/s, but you can see that it staggers a bit and even has a little episode of period increase (upward curvature near the beginning).