Optical, X-ray and radio observations of HD 61396: a probable new RS CVn-type binary

Abstract

We have carried out BVR photometric and Hα spectroscopic observations of the star HD 61396 during 1998 March 20 to 1999 April 3. We have discovered regular optical photometric variability from this star, with an inferred period of 31.95±0.10 d, and an amplitude of ~0.18 mag. A possible period of 35.34±0.12 d, as determined with Hipparcos, cannot be completely ruled out, however. Modelling of its photometric light curve with two circular spots indicates that 5-21 per cent of the stellar surface is covered by dark starspots which are ~830 K cooler than the surrounding photosphere, and produce the observed rotational modulation of the optical flux. Optical spectroscopy reveals a variable Hα emission feature, indicating that it is an unusually active star. In addition, we have analysed archival X-ray data of HD 61396, obtained from serendipitous observations with the ROSAT X-ray observatory, and we also discuss the radio properties of this star, based on both published Green Bank and unpublished VLA observations. The strong photometric variability and Hα emission, the relatively hard X-ray spectrum, and the high X-ray and radio luminosities imply that HD 61396 is most likely to be a member of the RS CVn class of evolved active binary stars. Its X-ray and radio luminosities place it among the five most luminous active binaries detected so far.