Astron. Astrophys. 338, 465-478 (1998)

1. Introduction

Magnetic cataclysmic variables (mCVs) (see Warner 1995 for a detailed description) are interacting low-mass binaries consisting of a late-type, mass donating secondary and an accreting white dwarf primary with a strong magnetic field. Among the magnetic cataclysmic variables two distinct subclasses are known: For the AM Herculis stars (or polars, MG) the rotation of the primary is locked to the orbital period and the matter is accreted along the field lines onto a small area near one (or both) magnetic poles without forming a disc. In DQ Herculis stars (or intermediate polars, IPs) on the other hand, the magnetic field is likely to be smaller, MG, the white dwarf rotates freely and accretion is in general fed via an intermediary accretion torus. Polars have typical periods below the period gap between 2 and 3 hours, the periods of IPs cluster above the gap. The combination of a lower magnetic field (on the average) together with higher mass accretion rate (on the average) leads to a break-up of spin-orbit synchronism for the case of IPs (Hameury et al. 1989).

RX J0203.8+2959 (henceforth referred to as RX J0203) was contained in a subsample of sources with a soft X-ray spectrum discovered during the ROSAT All-Sky Survey (RASS) and optically identified as an AM Her type object with a suspected period of 3.8 hr (Beuermann & Thomas 1993). In this paper we present observations that will show unequivocally that RX J0203 is an AM Herculis binary with a period of 4.6 hr. Thus it forms together with the hour system V1309 Ori ( RX J0515.6+0105) and V895 Cen ( EUVE J1429-38.0) ( hr) a group of outstanding systems with long orbital periods, which are important in order to investigate conditions and mechanisms that maintain synchronism.

© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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