GROND r′ band image of the region around 3XMM J051034.6−670353 obtained on 2016 August 4, with an exposure time of 2160 s. For each of the two XMM-Newton observations the X-ray positions as provided in the 3XMM-DR6 catalogue (yellow circles) and from our source detection with SAS 15 (red circles) are marked. The green circle is centred on the position we derived (SAS 15) from observation 0741800301 after astrometric alignment using three QSOs. For the other observation no astrometric correction was required. Positional errors (1σ) listed in Table1 are used for the circle radii. For the three brightest objects closest to the X-ray positions (labelled Star 1, 2, and 3, anticlockwise with increasing distance) photometry was conducted in the seven GROND bands.