Star HD 168625, Spitzer and WISE images: a) WISE 10′ × 10′ tricolour image R = WISE4, G = WISE2, B = WISE1. The intermediate and external nebulae are visible. The star ≃1′ west of HD 168625 is HD 168607. b) 2.75′ × 2.75′Spitzer-IRAC4 image. The image shows the nebular rings reported by Smith (2007). c) WISE channel 4 image at 22 μm of HD 168625 with labels of the internal, intermediate, and external shells (shown by ellipses). The position of the NE ring reported by Smith (2007) is also indicated. It matches the outer border of the intermediate shell and the inner border of the external shell. In these images, it is just possible to see a local flux minimum between the intermediate and external shell. This flux minimum is used to define the limits of both shells.