tokens: ACKNOWLEDGEMENTS This work has benefitted significantly from the leading contributions to our field over many years of three sadly departed colleagues: Neil Gehrels, Javier Gorosabel, and Peter Curran . The authors acknowledge useful discussions with Avi Loeb, Andrew Pontzen, Martin Haehnelt, and Alex de Koter. Partly based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 280.D-5059, 081.A-0856, 082.A-0301, 083.A-0644, 091.A-0442, 100.D-0649, and 102.D-0662. Partly based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de Canarias, under programs 31-014, 32-010, 39-023, 48-005, and 51-504. Partly based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofśica de Canarias, in the island of La Palma, Spain. Partly based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, under programs A26TAC_63 and A32TAC_5. The WHT and its override programme (for 090715B: programme W09AN001, P.I. Curran; for 161017A: programme W/2017A/23, P.I. Levan) are operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. We thank A. Kamble, R. Starling, and P. Curran for their help with the 090715B observations, and Marie Hrudkova for executing the 161017A observations. Partly based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). Partially based on data from the Grand Telescopio Canarias (GTC) Public Archive at the Centro de Astrobiología (CAB), under the Spanish Research Council (CSIC) and the National Institute of Technical Aerospace (INTA). This work made use of the GRBspec data base http://grbspec.iaa.es . JJ acknowledges support from Nederlandse Onderzoekschool Voor Astronomie (NOVA) and The Netherlands Organisation for Scientific Research (NWO) - The São Paulo Research Foundation (FAPESP) grant for advanced instrumentation in astronomy. KEH acknowledges support by a Project Grant (162948–051) from The Icelandic Research Fund. DAK acknowledges support from the Spanish research project AYA 2014-58381-P and Juan de la Cierva Incorporación IJCI-2015-26153. AJL and ERS acknowledge Science and Technology Facilities Council (STFC) consolidated grant ST/L000733/1. NRT and KW acknowledge Science and Technology Facilities Council (STFC) consolidated grant ST/N000757/1. AC acknowledges National Aeronautics and Space Administration (NASA) grant NNX15AP95A. AdUP acknowledges support from a Ramón y Cajal fellowship (RyC-2012-09975), a 2016 Banco Bilbao Vizcaya Argentaria (BBVA) Foundation Grant for Researchers and Cultural Creators, and from the Spanish research project AYA 2014-58381-P. RC acknowledges partial support from National Aeronautics and Space Administration (NASA) Swift grant NNX16AB04G. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The Sloan Digital Sky Survey-III (SDSS-III) web site is http://www.sdss3.org/ . SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofísica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 725246).
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