We announce the discovery that WASP-20 is a binary stellar system , consisting of two components separated by 0.2578 \pm 0.0007 ^ { \prime \prime } on the sky , with a flux ratio of 0.4639 \pm 0.0015 in the K -band .
It has previously been assumed that the system consists of a single F9 V star , with photometric and radial velocity signals consistent with a low-density transiting giant planet .
With a projected separation of approximately 60 au between the two components , the detected planetary signals almost certainly originate from the brighter of the two stars .
We reanalyse previous observations allowing for two scenarios , ‘ planet transits A ’ and ‘ planet transits B ’ , finding that both cases remain consistent with a transiting gas giant .
However , we rule out the ‘ planet transits B ’ scenario because the observed transit duration requires star B to be significantly evolved , and therefore have an age much greater than star A .
We outline further observations which can be used to confirm this finding .
Our preferred ‘ planet transits A ’ scenario results in the measured mass and radius of the planet increasing by 4 \sigma and 1 \sigma , respectively .