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Radial velocity variations in the young eruptive star EX Lup

EX Lup-type objects (EXors) are low-mass pre-main sequence objects characterized by outbursts attributed to highly enhanced disk accretion. The trigger mechanism of EXor outbursts is still debated. One theory requires a close (sub)stellar companion that perturbs the inner disk and triggers the onset of the outburst. Here, we study the radial velocity (RV) variations of EX Lup, the prototype of EXors. We conducted a 5-year RV survey with HARPS and FEROS. We analyzed the activity of EX Lup by checking the bisector and several other activity indicators. We complemented the RV data with photometric monitoring to look for signatures of activity or varying accretion. We found that the RV of EX Lup is periodic, with stable period, semi-amplitude, and phase over at least four years of observations. This period is not present in any of the activity indicators. However, the RV of narrow metallic emission lines suggest the same period, but with an anti-correlating phase. The observed absorption line RVs can be fitted with a Keplerian solution around a 0.6 M_Sun central star with M2 sin i = (14.7 +/- 0.7) M_Jup and eccentricity of e = 0.24. Alternatively, we attempted to model the observations with a cold/hot stellar spot as well, but the spot parameters needed to reproduce the RV semi-amplitude are in contradiction with the photometric variability, making the spot scenario unlikely. We discuss two possibilities to explain the RV data: a geometry with two accretion columns rotating with the star, and a single accretion flow synchronized with the orbital motion of the hypothetical companion. In the companion scenario, the companion's mass would fall into the brown dwarf desert, which, together with the unusually small separation would make EX Lup a unique binary system, with interesting implications on the physical mechanisms responsible for triggering the outburst.