HS 1023 + 3900 – a magnetic CV in the period gap with a distinct cyclotron emission line spectrum ?

We report on the discovery by the Hamburg Quasar Survey of a magnetic CV with a pure cyclotron emission line spectrum. With a photometric and spectroscopic period of 167.6 min it is located in the CV period gap. From 91 spectra taken over 1.2 orbital periods we identify the n = 2 and 3 cyclotron harmonics of the main accretion pole with 60 MG as well as the n = 2 and 3 modes of the secondary pole with 68 MG. HS 1023+3900 has not been seen as an X-ray source by ROSAT. The estimated accretion rate is with ˙ M < 3 · 10 −13 M yr −1 at least two orders of magnitude lower than normally observed in polars. The observations of a pure cyclotron spectrum together with a low specific accretion rate (< 10 −3 g cm −2 s −1) are consistent with theoretical computations of cyclotron spectra of magnetic white dwarfs based on the so called " bombardment solution " by Rousseau et al. (1996) which predict just such behaviour. The location in the period gap is also consistent with an extremely low accretion rate according to standard scenarios for the evolution of CVs.