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 HD 216803 , 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 _ { \mathrm { 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 \sim 4 M _ { J } ) companion in a relatively close ( a \sim 1 AU ) 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 _ { \mathrm { 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 .