We present the first in a series of papers that attempt to investigate the
relation between binarity, magnetic activity, and chemical surface abundances
of cool stars. In the current paper, we lay out and test two abundance analysis
methods and apply them to two well-known, active, single stars, HD 27536
(G8IV-III) and HD 216803 (K5V), presenting photospheric fundamental parameters
and abundances of Li, Al, Ca, Si, Sc, Ti, V, Cr, Fe, Co and Ni. The abundances
from the two methods agree within the errors for all elements except calcium in
\hdeen, which means that either method yields the same fundamental model
parameters and the same abundances. Activity is described by the radiative loss
in the Ca II H & K lines with respect to the bolometric luminosity, through the
activity index R_{HK}. Binarity is established by very precise radial velocity
(RV) measurements using HARPS spectra. The spectral line bisectors are examined
for correlations between RV and bisector shape to distinguish between the
effects of stellar activity and unseen companions. We show that HD 27536
exhibit RV variations mimicking the effect of a low-mass (m ~ 4M_J) companion
in a relatively close (a ~ 1AU) orbit. The variation is strongly correlated
with the activity, and consistent with the known photometric period P = 306.9
d, demonstrating a remarkable coherence between R_{HK} and the bisector shape,
i.e. between the photosphere and the chromosphere. We discuss the complications
involved in distinguishing between companion and activity induced RV
variations.Comment: 11 pages, 12 figures. A&A accepte
