A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris

Aims. We searched for long-term period changes in the polar EK UMa using new optical data and archival X-ray/EUV data. Methods. An optical ephemeris was derived from data taken remotely with the MONET/N telescope and compared with the X-ray ephemeris based on Einstein, ROSAT ,a ndEUVE data. A three-parameter fit to the combined data sets yields the epoch, the period, and the phase offset between the optical minima and the X-ray absorption dips. An added quadratic term is insignificant and sets a limit to the period change. Results. The derived linear ephemeris is valid over 30 years and the common optical and X-ray period is P = 0.0795440225(24) days. There is no evidence of long-term O − C variations or a period change over the past 17 years (ΔP = −0.14 ± 0.50 ms). We suggest that the observed period is the orbital period and that the system is tightly synchronized. The limit on ΔP and the phase constancy of the bright part of the light curve indicate that O − C variations of the type seen in the polars DP Leo and HU Aqr or the pre-CV NN Ser do not seem to occur in EK UMa. The X-ray dips lag the optical minima by 9.5 ◦ ± 0.7 ◦ in azimuth, providing some insight into the accretion geometry.