1998A&A...330.1029W

Multiwavelength Doppler images of the long-period RS CVn binary IL Hydrae from March 1994 and Feb.-March 1995 reveal a cool polar spot and several features at low latitudes. Their average temperature is approximately 500K below the photospheric temperature of 4700K. Due to the relatively small rotational velocity of 26.5±1km/s and the relatively cool, low-gravity atmosphere with many weak absorption blends, Doppler imaging of IL Hydrae is challenging but resulted in encouragingly similar maps from altogether seven spectral regions near 6400Å. Latitude-dependent differential rotation is detected from our Doppler maps and indicate faster rotation at the stellar equator as compared to the polar regions. Lines from the secondary component are sometimes seen in our red-wavelength spectra and a double-lined spectroscopic orbit is presented. We also give a more precise rotational period from three consecutive years of V-band photometry. A spectral classification of K0III-IV and a mass of 2.2±0.2M_☉ for the primary, and mid to late F main sequence or maybe ≃G0V-IV and a mass of 1.3±0.2M_☉ for the secondary, are suggested. Hα equivalent widths measured over a single rotation cycle exhibit a sinusoidal variation in phase with the photospheric V-band light curve. The maximum of the residual Hα emission coincides with the lightcurve minimum and is thus likely linked to the starspot-covered stellar surface. A steady, redshifted absorption component possibly indicates an isotropic inward flow at the Hα line-forming region at approximately 60km/s.