Context : Short- to mid-term magnetic phenomena on the stellar surface of M-type stars can not only resemble the effects of planets in radial velocity data , but also may hide them .

Aims : We analyze 145 spectroscopic HARPS-N observations of GJ 3942 taken over the past five years and additional photometry to disentangle stellar activity effects from genuine Doppler signals as a result of the orbital motion of the star around the common barycenter with its planet .

Methods : To achieve this , we use the common methods of pre-whitening , and treat the correlated red noise by a first-order moving average term and by Gaussian-process regression following an MCMC analysis .

Results : We identify the rotational period of the star at 16.3 days and discover a new super-Earth , GJ 3942 b , with an orbital period of 6.9 days and a minimum mass of 7.1 M _ { \oplus } .
An additional signal in the periodogram of the residuals is present but we can not claim it to be related to a second planet with sufficient significance at this point .
If confirmed , such planet candidate would have a minimum mass of 6.3 M _ { \oplus } and a period of 10.4 days , which might indicate a 3:2 mean-motion resonance with the inner planet .

Conclusions :