Doppler imaging of stellar surface structure - V. The long-period RS CVn binary HD 81410=IL hydrae

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 500 K below the photospheric temperature of 4700 K. Due to the relatively small rotational velocity of 26.5+/-1 km s(-1) 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 6400 Angstrom. 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 KOIII-IV and a mass of 2.2+/-0.2 M-circle dot for the primary, and mid to late F main sequence or maybe approximate to GOV-IV and a mass of 1.3+/-0.2 Mo for the secondary, are suggested. H alpha 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 H alpha 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 Her line-forming region at approximately 60 km s(-1).