We present the discovery of NGTS-3Ab , a hot Jupiter found transiting the primary star of an unresolved binary system .
We develop a joint analysis of multi-colour photometry , centroids , radial velocity ( RV ) cross-correlation function ( CCF ) profiles and their bisector inverse slopes ( BIS ) to disentangle this three-body system .
Data from the Next Generation Transit Survey ( NGTS ) , SPECULOOS and HARPS are analysed and modelled with our new blendfitter software .
We find that the binary consists of NGTS-3A ( G6V-dwarf ) and NGTS-3B ( K1V-dwarf ) at < 1 \arcsec separation .
NGTS-3Ab orbits every 1.675 days .
The planet radius and mass are R _ { \mathrm { planet } } = 1.48 \pm 0.37 R _ { J } and M _ { \mathrm { planet } } = 2.38 \pm 0.26 M _ { J } , suggesting it is potentially inflated .
We emphasise that only combining all the information from multi-colour photometry , centroids and RV CCF profiles can resolve systems like NGTS-3 .
Such systems can not be disentangled from single-colour photometry and RV measurements alone .
Importantly , the presence of a BIS correlation indicates a blend scenario , but is not sufficient to determine which star is orbited by the third body .
Moreover , even if no BIS correlation is detected , a blend scenario can not be ruled out without further information .
The choice of methodology for calculating the BIS can influence the measured significance of its correlation .
The presented findings are crucial to consider for wide-field transit surveys , which require wide CCD pixels ( > 5 \arcsec ) and are prone to contamination by blended objects .
With TESS on the horizon , it is pivotal for the candidate vetting to incorporate all available follow-up information from multi-colour photometry and RV CCF profiles .