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tokens: models that terminate at values in agreement with the inferred luminosity and temperature for SN 2016gkg (as discussed in Section 3.4.2 ). The best-fitting model has an initial stellar mass of M 15M with a 1.5-M companion. The initial period is 1000d and the primary star explodes with M 5.2M . Two additional examples with initial masses 12M (8.4-M initial mass companion, 160-d period) and 16M (14.4-M initial mass companion, 6.3-d period) are shown with dotted and dashed lines, respectively. All of these models agree with the inferred luminosity and temperature of SN 2016gkg. We find that it is extremely unlikely that SN 2016gkg originated from a single star. Woosley Heger ( 2007 ) and Sukhbold etal. ( 2016 ) have found that, for stars with M ZAMS 30M , the pre-SN iron core is too large for an SN to be successful. Moreover, mass-loss is sufficiently strong so that most of these stars lose their entire hydrogen envelopes and are thought to end their evolution as WolfRayet stars, implying that the subsequent SN would be Type Ib or Ic. SNe IIb require progenitor stars with extended low-mass hydrogen envelopes (Podsiadlowski 1993 ; Woosley etal. 1994 ; Elmhamdi etal. 2006 ), and any single-star model for such a system would require finely tuned mass-loss that would otherwise fail to reproduce the observed range in SN IIb light curves, spectra, and progenitor stars. While the single-star scenario could describe a minority of SNe IIb, it is likely that the majority of these systems come from binary-star systems such as the one observed towards SN 1993J (Maund etal. 2004 ; Fox etal. 2014 ). 3.4.2 Binary-star models We examine evolutionary tracks involving binary stars in order to assess the plausibility of these systems as possible progenitor stars for SN 2016gkg. We obtained our binary-star evolutionary tracks from the Binary Population and Spectral Synthesis ( bpass ) code as described in Eldridge Stanway ( 2009 ). These models provide a range of metallicities ( Z 0.0010.040), primary-star masses ( M /M 0.1300), mass ratios ( q 0.10.9), and initial periods [log( P /1d)04]. We fixed the metallicity of the binary-star models to Z 0.010 in order to provide the best match to the observed metallicity of NGC 613. Otherwise, we examined the full range of parameters provided by bpass . For our fitting scheme, we looked for binary-star models that produced a primary star with terminal luminosity and temperature that minimized Overall, we found that 107 out of 5565 bpass models terminated with primary-star luminosity and temperature within our measurement uncertainties for the SN 2016gkg progenitor star. Of these models, the 2 ranged from 0.11 to 4.01 with a median value of 1.05. In Fig. 4 , we show the stellar evolution of the best-fitting binary-star model on the HR diagram along with the inferred luminosity and temperature of the SN 2016gkg progenitor star. The primary star has an initial mass of 15M while the secondary (accreting) star has an initial