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tokens: first mechanism does not necessarily lead to an immediate terminal SN explosion, it may explain the three-decade-long eruptive phase of SN 2000ch ( Pastorello et al. 2010 ). The second mechanism is promising as it may reproduce the sequence of events leading to SN 2009ip and its subsequent evolution (e.g. Graham et al. 2017 , and references therein). Alternatively, mass loss, driven by waves generated in the convective core during the late nuclear burnings, has also been proposed for moderate-mass ( M ZAMS ∼ 20 M ⊙ ) stars and for very massive stars at low-metallicity regime. This would explain the violent mass ejections observed in some SNe IIn from a decade to a few months prior to core-collapse ( Quataert Shiode 2012 ; Shiode Quataert 2014 ). The modern strategy of SN searches and the availability of big data archives have increased the discoveries of pre-SN outbursts, but the number of events is still of the order of a few tens, and is limited in volume. Future SN searches with big telescopes, in particular the Large Synoptic Survey Telescope ( LSST Science Collaboration 2009 ), will enormously increase the discovery rates, and will provide us with valuable datasets of light curves with coverage of many years. Appendix A: Optical photometry of SN 2018cnf Table A.1. Optical photometry of SN 2018cnf: Johnson-Bessell B , V (Vega mag), Sloan u , g , r , i , z and PanSTARRS y (AB mag). Notes. PS1 = 1.8 m Pan-STARRS Telescope + GPC1 camera (Haleakala, Hawaii Islands, USA); ATLAS = 0.5 m ATLAS-1 Telescope + STA-1600 CCD (Haleakala, Hawaii Islands, USA); ASAS-SN-5 = 0.16 m Payne-Gaposchkin Telescope + FLI ProLine PL230 CCD (Las Cumbres Observatory – South African Astronomical Observatory); ASAS-SN-3 = 0.16 m Paczyński Telescope + FLI ProLine PL230 CCD (Las Cumbres Observatory – Cerro Tololo Inter-American Observatory, Chile); GTC = 10.4 m Gran Telescopio Canarias + OSIRIS (La Palma, Canary Islands, Spain); LT = 2.0 m Liverpool Telescope + IO:O (La Palma, Canary Islands, Spain); fl11 = 1.0 m Telescope (Dome 3) + Sinistro CCD (Las Cumbres Observatory – Siding Spring Observatory, Australia); fl16 = 1.0 m Telescope (Dome 5) + Sinistro CCD (Las Cumbres Observatory – South African Astronomical Observatory); fl03 = 1.0 m Telescope (Dome 4) + Sinistro CCD (Las Cumbres Observatory – Cerro Tololo Inter-American Observatory, Chile); fl15 = 1.0 m Telescope (Dome 2) + Sinistro CCD (Las Cumbres Observatory – Cerro Tololo Inter-American Observatory, Chile); fl06 = 1.0 m Telescope (Dome 7) + Sinistro CCD (Las Cumbres Observatory – South African Astronomical Observatory); fl12 = 1.0 m Telescope (Dome 8) + Sinistro CCD (Las Cumbres Observatory – Siding Spring Observatory, Australia); Copernico = 1.82 m Copernico Telescope + AFOSC (Mt. Ekar, Asiago Observatory, Italy); NTT = 3.58 m New Technology Telescope + EFOSC2 (ESO-La Silla, Chile); NOT = 2.56 m Nordic Optical telescope + ALFOSC (La Palma, Canary Islands, Spain). ( ‡ ) PanSTARRS w band converted to Sloan r ,