DW cancri: a magnetic VY Scl star with an orbital period of 86 min

ABSTRACT We present the first time-resolved spectroscopic study of the cataclysmic variableDWCancri. We have determined an orbital period of 86.10 ± 0.05min, which placesthe system very close to the observed minimum period of hydrogen-rich cataclysmicvariables. This invalidates previous speculations of DW Cnc being either a permanentsuperhumper below the period minimum or a nova-like variable with an orbital periodlonger than 3 hours showing quasi-periodic oscillations. The Balmer and Hei lineshave double-peaked profiles and exhibit an intense S-wave component moving withthe orbital period. Remarkably, the Balmer and Hei radial velocity curves are modu-lated at two periods: 86.10±0.05min (orbital) and 38.58±0.02min. The same shortperiod is found in the equivalent width variations of the single-peaked Heii λ4686line. We also present time-resolved photometry of the system which shows a highly-coherent variation at 38.51 min, consistent with the short spectroscopic period. Thelarge number of similarities with the short-period intermediate polar V1025Cen leadus to suggest that DWCnc is another intermediate polar below the period gap, and wetentatively identify the photometric and spectroscopic 38min signals with the whitedwarf spin period. DWCnc has never been observed to undergo an outburst, but itoccasionally exhibits low states ∼ 2 mag fainter than its typical brightness level ofV ≃ 14.5, resembling the behaviour of the high mass-transfer VYScl stars.Key words: accretion, accretion discs – binaries: close – stars: individual: DW Cnc– novae, cataclysmic variables