  Aim. The unusual transient ATLAS17aeu was serendipitously detected within the
sky localisation of the gravitational wave trigger GW170104. The importance of
a possible association with gravitational waves coming from a binary black hole
merger led to an extensive follow-up campaign, with the aim of assessing a
possible connection with GW170104. Methods. With several telescopes, we carried
out both photometric and spectroscopic observations of ATLAS17aeu, for several
epochs, between $\sim 3$ and $\sim 230$ days after the first detection.
Results. We studied in detail the temporal and spectroscopic properties of
ATLAS17aeu and its host galaxy. We detected spectral features similar to those
of a broad lined supernova superposed to an otherwise typical long-GRB
afterglow. Based on analysis of the optical light curve, spectrum and host
galaxy SED, we conclude that the redshift of the source is probably $z \simeq
0.5 \pm 0.2$. Conclusions. While the redshift range we have determined is
marginally compatible with that of the gravitational wave event, the presence
of a supernova component and the consistency of this transient with the E$_{\rm
p}$-E$_{\rm iso}$ correlation support the conclusion that ATLAS17aeu was
associated with the long gamma-ray burst GRB170105A. This rules out the
association of the GRB170105A/ATLAS17aeu transient with the gravitational wave
event GW170104, which was due to a binary black hole merger.
