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tokens: We thank the referee. We acknowledge ESA Gaia ( http://www.cosmos.esa.int/gaia ), DPAC ( http://www.cosmos.esa.int/web/gaia/dpac ) and the DPAC Photometric Science Alerts Team ( http://gaia.ac.uk/selected-gaia-science-alerts ). This work was partly supported by the European Union FP7 programme through ERC grant nos. 320360 and 320964 (WDTracer). TRM, EB and DS acknowledge support from the UK STFC in the form of a Consolidated Grant #ST/L00073. AH acknowledges support from the Leverhulme Trust through grant RPG-2012-541. MT, LT and PO are partially supported by the PRIN-INAF 2014 Transient Universe: unveiling new types of stellar explosions with PESSTO. The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement no. 264895. We acknowledge support from the Polish NCN grant 2012/06/M/ST9/00172 to LW, OPTICON FP7 EC grant no. 312430 and Polish MNiSW W32/7.PR/2014 grant to LW. TW-SH is supported by the DOE Computational Science Graduate Fellowship, grant no. DE-FG02-97ER25308. Support for JLP is provided in part by FONDECYT through the grant 1151445 and by the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. GD and OV gratefully acknowledge the observing grant support from the Institute of Astronomy and Rozhen NAO BAS; this work is in line with the Projects no. 176011, no. 176004 and no. 176021 supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia. BJS is supported by NASA through Hubble Fellowship grant HF-51348.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. PAW thanks the National Research Foundation for a research grant. We acknowledge support from the Polish NCN grant 2011/03/B/ST9/02667 to ZK. We thank the Comité Científico Internacional (CCI) of the European Northern Observatory (ENO) in the Canary Islands, for awarding time for this project under the 5 per cent International Time Programme (ITP). PS1 is run by the Institute for Astronomy, University of Hawaii, Pan-STARRS Project Office, Max Planck Institute for Astronomy, Heidelberg, Max Planck Institute for Extraterrestrial Physics, Garching, Johns Hopkins University, Durham University, University of Edinburgh, Queen's University Belfast, Harvard–Smithsonian Center for Astrophysics, Las Cumbres Observatory Global Telescope Network Incorporated, National Central University of Taiwan, Space Telescope Science Institute, NSF Grant AST-1238877, the University of Maryland, and Eotvos Lorand University, Los Alamos National Laboratory, and NASA Grants NNX08AR22G issued by the Planetary Science Division of the NASA Science Mission Directorate, and grants NNX12AR65G, NNX14AM74G issued through the NEO Observation Program. The CSS survey is funded by NASA under Grant No. NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. The CRTS survey is supported by the US National Science Foundation under grants AST-0909182 and AST-1313422. The WHT is operated by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos (Proposal P29). The Copernico 1.82 m telescope is operated by INAF Osservatorio Astronomico di Padova. The Cassini 1.52 m telescope is operated by INAF Osservatorio Astronomico di Bologna.