1 Introduction

DP Leo is one of the strongly magnetic cataclysmic binaries of AM Her type, a so called polar. It was discovered as the first eclipsing polar some 20 years ago as optical counterpart of the EINSTEIN source E1114+182 (Biermann et al. 1985), and continuously observed from the ground and the space with e.g. HST (Stockman et al. 1994) and ROSAT (Robinson & Cordova 1994, hereafter RC94). It was found to be a two-pole accretor based on the detection of cyclotron emission lines in field strengths of 30.5 MG and 59 MG, respectively (Cropper & Wickramasinghe 1993). A thorough timing study by Robinson & Cordova (1994) using ROSAT X-ray data combined with earlier optical data revealed evidence for an asynchronous rotation of the white dwarf in the system. Asynchronous polars form a very small subgroup of all polars. There are four out of currently known 65 systems which show a small degree of asynchronism of typically about 1% (Campbell & Schwope 1999). With an extra spin of the white dwarf in DP Leo of about 2$^\circ$-2.5$^\circ$ per year, the degree of asynchronism is seemingly much smaller than in the other four objects. However, the earlier results are based on a mixture of optical and X-ray data with not necessarily common origin on the white dwarf.

DP Leo was chosen as Calibration/Performance Verification target of XMM-Newton and was observed with all three X-ray telescopes in Nov. 2000. The spectrum derived from these observations was published by Ramsay et al. (2001). Using a multi-temperature model of the post-shock flow, they found evidence of a very massive white dwarf in excess of 1 $M_\odot$. In order to address the question of asynchronism in DP Leo based on X-ray data alone, we performed a timing analysis of the new XMM-data in combination with archival ROSAT observations (one published by Robinson & Cordova, a second one unpublished, Sect. 2). For proper measurement of the eclipse parameters we used segmented data, where individual segments were determined with a Bayesian change point detection method (Sect. 2.3). Our main results are presented in Sect. 3, where the eclipse parameters, an updated eclipse ephemeris and the accretion geometry are discussed. The question whether there is a positive detection of the secondary at X-ray wavelengths in the eclipse is discussed in Sect. 3.7.

After acceptance of a paper with our original analysis (astro-ph/0111457), we were made aware of an inconsistency between the XMM-Newton timing system as documented (UTC) and as actually used (TT, D. Pandel, private communication). The difference between the two time frames at the time of the XMM-Newton observation was 63.184 s. The shift of the derived eclipse time made a re-determination of our original eclipse ephemeris necessary. As a consistency check for the new ephemeris, we performed high-speed photometric optical observations yielding an additional epoch for the eclipse center in January 2002. Ultraviolet and optical observations are described in Sects. 2.4 and 2.5, the analysis of these data is described in Sects. 3.2 and 3.3.