Polarimetric and spectroscopic optical observations of the ultra-compact X-ray binary 4U 0614+091

Aims. We present a polarimetric and spectroscopic study of the persistent ultra-compact X-ray binary 4U 0614+091 aimed at searching for the emission of a relativistic particle jet and at unveiling the orbital period P-orb of the system. Methods. We obtained r-band polarimetric observations with the Telescopio Nazionale Galileo (TNG) equipped with the PAOLO polarimeter and with the Nordic Optical Telescope (NOT) equipped with the ALFOSC instrument, covering similar to 2 h and similar to 0.5 h observations, respectively. We carried out low resolution spectroscopy of the system using the ESO Very Large Telescope equipped with FORS1 for similar to 1.5 h (16 spectra covering the range 4300 8000 angstrom). Results. The polarimetric analysis performed starting from the TNG dataset revealed a polarisation degree in the r-band of 3% +/- 1%. From the NOT dataset, due to the lower signal-to-noise ratio, we could obtain only a 3 sigma upper limit of 3.4%. From the joining of a spectroscopic and photometric analysis, through the study of the equivalent width variations of the CII 7240 angstrom line and the r-band light curve, we could find a hint of a similar to 45 min periodicity. Conclusions. A polarisation degree P of similar to 3% in the r-band is consistent with the emission of a relativistic particle jet, which is supposed to emit intrinsically linearly polarised synchrotron radiation. Since no variations of P with time have been detected, and the accretion disc of the system does not contain ionised hydrogen, scattering by free electrons in the accretion disc has been rejected. The period of similar to 45 min obtained through the analysis of the system light curve and of the equivalent width variations of the selected spectral line is probably linked to the presence of a hot spot or a superhump in the accretion disc, and lead to an orbital period greater than or similar to 1 h for the binary system.