The latest three WASP-South planets appeared on the preprint server arXiv today, announcing WASP-68b, WASP-73b and WASP-88b.

The paper is led by the University of Liège, who operate the TRAPPIST robotic 0.6-m photometer, sited at La Silla in Chile. TRAPPIST plays a crucial role in WASP-South planet discovery. Since the WASP photometry, from 200mm, f/1.8 lenses, is relatively crude, a TRAPPIST lightcurve of a candidate gives a better idea of whether the candidate is worth pursuing. Further, the large pixels of WASP data mean that apparent transits are often caused by deeply eclipsing fainter stars within the WASP photometry aperture. TRAPPIST photometry shows up such blends, and thus avoids wasting valuable radial-velocity observations.

As an example, the candidate 1SWASPJ113725.66–261925.6 showed a shallow 1% dip recurring with a period of 1.33 days, possibly caused by a planet transit. However, close by are two fainter stars (left, the yellow circle is the extraction aperture used for the WASP photometry; WASP pixels are much bigger than in this Sky Survey image, and so the candidate and the two close stars are merged).

TRAPPIST is able to resolve the stars, and followup photometry showed that the dip is caused by the closest faint star, which is an eclipsing binary with a 20% deep eclipse (the image is reversed left–right). In photometry centered on the bright candidate this eclipse is diluted to a shallow planet-like dip.