We present in this paper a 6 yr time series of magnetic (and brightness)
surface images of the K1 subgiant of the RS CVn system HR 1099 (=V711 Tauri)
and of the young K0 dwarf LQ Hydrae, reconstructed (with the help of a
dedicated maximum entropy image reconstruction software) from Zeeman-Doppler
imaging observations collected at the Anglo-Australian Telescope.
All stellar magnetic images we reconstruct host at least one high contrast
feature in which the field is predominantly azimuthal, thus confirming that
such surface structures (already detected in previous similar studies) are
indeed real. We take this as strong evidence that dynamo processes of
late-type rapid rotators operate throughout the whole stellar convective
envelope, rather than being confined to an interface layer between the
convective and radiative zones as in the Sun. The latitudinal polarity
pattern of azimuthal and radial fields that we observe at the surface of
both stars suggest that these magnetic regions respectively reveal the
toroidal and poloidal components of the large scale dynamo field. The
spatial structure of these two magnetic field components gets increasingly
more complex (with higher axisymmetric spherical harmonic degrees) for
larger rotation rates and deeper convective zones. The strength of the
toroidal and poloidal components is typically a few hundred G, more than
two orders of magnitude stronger than in the Sun. Long-term evolution
of the toroidal and poloidal components of the large scale field is clearly
detected in our time series.
We also report the detection of small fluctuations in the orbital period
of HR 1099, with a peak-to-peak amplitude of about 36+-1 s (i.e. 0.015%)
and a period of about 18+-2 yr (assuming sinusoidal variations around the
nominal value). The most plausible way of explaining such fluctuations
is that the quadrupole moment of the K1 subgiant is varying with time, and
that this modulation is driven by the magnetic activity cycle of the primary
star itself (through periodic exchange between kinetic and magnetic energy).
It provides in particular an independent confirmation that dynamo operates
within the whole convective zone, and suggests that the average azimuthal
field in the convective envelope is of the order of 6 kG.