Context : Multi-wavelength time-series observations with high cadence and long duration are needed to resolve and understand the many variations of magnetically active late-type stars , which is an approach often used to observe the Sun .

Aims : We present a first and detailed study of the bright and active K0IV-III star HD 123351 .

Methods : We acquired a total of 955 high-resolution STELLA echelle spectra during the years 2006–2010 and a total of 2,260 photometric VI _ { C } data points during 1998–2010 .
These data are complemented by some spectra from CFHT and KPNO .

Results : The star is found to be a single-lined spectroscopic binary with a period of 147.8919 \pm 0.0003 days and a large eccentricity of e =0.8086 \pm 0.0001 .
The rms of the orbital solution is just 47 m s ^ { -1 } , making it the most precise orbit ever obtained for an active binary system .
The rotation period is constrained from long-term photometry to be 58.32 \pm 0.01 days .
It shows that HD 123351 is a very asynchronous rotator , rotating five times slower than the expected pseudo-synchronous value .
Two spotted regions persisted throughout the 12 years of our observations .
We interpret them as active longitudes on a differentially rotating surface with a \Delta P / P of 0.076 .
Four years of H \alpha , Ca ii H & K and He i D3 monitoring identifies the same main periodicity as the photometry but dynamic spectra also indicate that there is an intermittent dependence on the orbital period , in particular for Ca ii H & K in 2008 .
Line-profile inversions of a pair of Zeeman sensitive/insensitive iron lines yield an average surface magnetic-flux density of 542 \pm 72 G. The time series for 2008 is modulated by the stellar rotation as well as the orbital motion , such that the magnetic flux is generally weaker during times of periastron and that the chromospheric emissions vary in anti-phase with the magnetic flux .
We also identify a broad and asymmetric lithium line profile and measure an abundance of \log n ( { Li } ) = 1.70 \pm 0.05 .
The star ’ s position in the H-R diagram indicates a mass of 1.2 \pm 0.1 M _ { \odot } and an age of 6-7 Gyr .

Conclusions : We interpret the anti-phase relation of the magnetic flux with the chromospheric emissions as evidence that there are two magnetic fields present at the same time , a localized surface magnetic field associated with spots and a global field that is oriented towards the ( low-mass ) secondary component .
We suggest that the inter-binary field is responsible for the magnetic-flux dilution at periastron .
It is also likely to be responsible for the unexpected slow and asynchronous rotation of the primary star .