A Refined Orbital Ephemeris for the Intermediate Polar FO Aquarii and Evidence for a Period Derivative

Abstract We combine more than 40 yr of eclipse timings of the intermediate polar FO Aqr and derive new orbital ephemerides. The eclipse times are best fit with a second order polynomial, implying an 8 σ detection of a period derivative. The results show that the orbital period of FO Aqr is currently increasing, with a derivative of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>˙</mml:mo> </mml:mrow> </mml:mover> <mml:mo>=</mml:mo> <mml:mn>6.65</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.81</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11</mml:mn> </mml:mrow> </mml:msup> </mml:math> .