Lluis is a member of the ePESSTO collaboration which followed up the astronomical transient resulting from the merger of two low-mass neutron stars in the galaxy NGCC 4993 on August 17th 2017. ePESSTO runs the 3.58m New Technology Telescope (NTT) at La Silla Observatory for 10 nights per month, divided in 3 runs of 4+3+3 nights. They took the first optical spectra during the first day of the first subrun of August (18th), the day after the discovery of the event (+1.48 days), and three more spectra the following three nights (19-21th). In addition, two simultaneous Ultraviolet/Optical/Near-infrared spectra were also taken on the second and fourth night with X-Soooter at the 8.1m Very Large Telescope al Cerro Paranal Observatory. Coordinated optical (Ugrizy) and Near-infrared (JHKs) imaging was performed with NTT, GROND and PanStarrs. The resulting light-curve and spectral sequence is presented in a Nature paper: Smartt et al (2017) http://www.nature.com/nature/journal/vaap/ncurrent/full/nature24303.html?foxtrotcallback=true
This transient did not look like any known supernova type. It evolved very fast in a few-days scale from a blue 7000K featureless continuum to a red 4000K continuum with two broad Caesium Cs I at 7400 A and Tellurium Te I at 8300 A features. When compared to a series of physical models, light-curve and spectral best fits were found for low-mass (0.04 Msun) high-velocity ejecta with an opacity consistent with a blend of elements with atomic masses in the 90–140 range, and powered by a source which was constrained to have a power law slope of −1.2, consistent with the r-process decay timescales. All these results made us conclude that the object was consistent with a blue-kilonova resulting of a neutron-star merger.