Unveiling the enigma of ATLAS17aeu

AM acknowledges the support from the ASI grant I/004/11/3. AR acknowledges support from Premiale LBT 2013. AJCT thanks the Spanish Ministry Project AYA2015-71718-R (including FEDER funds). LT and SB are partially supported by the PRIN-INAF 2016 with the project “Toward the SKA and CTA era: discovery, localisation, and physics of transient sources”. Partially based on observations collected at the Copernico 1.82 m telescope (Asiago, Italy) of the INAF - Osservatorio Astronomico di Padova. JH was supported by a VILLUM FONDEN Investigator grant (project number 16599). AFV is thankful to the Russian Science Foundation (grant 14-50-00043). This work made use of observations obtained with the Italian 3.6 m Telescopio Nazionale Galileo (TNG) and the 10.4 m Gran Telescopio Canarias (GTC), operated on the island of La Palma by the Fundación Galileo Galilei of the Instituto Nazionale di Astrofisica (INAF) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and also made use of observations obtained with the 8.4 m Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in Italy, the United States, and Germany. LBT Corporation partners are: Istituto Nazionale di Astrofisica, Italy; The University of Arizona on behalf of the Arizona university system; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University; and The Research Corporation on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia. We thank the TNG staff, in particular G. Andreuzzi, G. Mainella, A. Harutyunyan, and the LBT staff, in particular A. Gargiulo, for their valuable support with TNG and LBT observations and data reduction. We also acknowledge INAF financial support of the project “Gravitational Wave Astronomy with the first detections of adLIGO and adVIRGO experi