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

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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, [FORMULA] 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, [FORMULA] 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 [FORMULA]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 [FORMULA] hour system V1309 Ori ([FORMULA] RX J0515.6+0105) and V895 Cen ([FORMULA] EUVE J1429-38.0) ([FORMULA] hr) a group of outstanding systems with long orbital periods, which are important in order to investigate conditions and mechanisms that maintain synchronism.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998