Starship Asterisk*

[i]VLT Image of the Planetary Nebula Henize 2-428 [b](Credit: ESO)[/b][/i]

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First pair of merging stars destined to become supernova found

Astronomers using ESO facilities in combination with telescopes in the Canary Islands have identified two surprisingly massive stars at the heart of the planetary nebula Henize 2-428. As they orbit each other the two stars are expected to slowly get closer and closer, and when they merge, about 700 million years from now, they will contain enough material to ignite a vast supernova explosion. ...

The team of astronomers ... has discovered a close pair of white dwarf stars — tiny, extremely dense stellar remnants — that have a total mass of about 1.8 times that of the Sun. This is the most massive such pair yet found and when these two stars merge in the future they will create a runaway thermonuclear explosion leading to a Type Ia supernova.

The team who found this massive pair actually set out to try to solve a different problem. They wanted to find out how some stars produce such strangely shaped and asymmetric nebulae late in their lives. One of the objects they studied was the unusual planetary nebula known as Henize 2-428. ...

• Nature (online 09 Feb 2015) DOI: 10.1038/nature14124 (pdf)

From the Wikipedia article on Type 1a Supernova, linked to in the ESO science release excerpted in the opening post:
Double degenerate scenarios raise questions about the applicability of Type Ia supernovae as standard candles, since total mass of the two merging white dwarfs varies significantly, meaning luminosity also varies.