We report on the redshift of the BL Lac object 3FGL J0909.0+2310 based on observations obtained with the OSIRIS Multi Object Spectrograph ( MOS ) mounted on the 10.4-m Gran Telescopio Canarias .
A redshift of 0.432 \pm 0.002 was obtained by the identification of three absorption features ( Ca II K & H and G-band ) detected in the spectrum of the BL Lac host galaxy .
The closest object to the BL Lac at an angular separation of 3.8 \arcsec ( \sim 21 kpc at this distance ) has a similar redshift of 0.431 \pm 0.002 .
This companion galaxy could be the most likely cause of the nuclear activity as postulated by studies based on more extended data sets and cosmological models .
MOS allows us to study the object ’ s neighbourhood within a field of view of approximately 7 ’ \times 2 ’ and we find two small groups of galaxies at redshifts 0.28 and 0.39 which are probably not related to the activity of 3FGL J0909.0+2310 .