False-colour HST ACS image of the area around the ULX Ho II X-1 with the adjacent H II region known as the ‘Foot Nebula’. This composite image comprises observations taken with the F502N (blue), F658N (green), and F814W (red) filters. The white circle highlights the He III bubble generated by ionising X-ray radiation emitted by the ULX, and has a diameter of 2$\stackrel{″}{.}$5, corresponding to 37 pc at the distance of the Holmberg II galaxy. The ULX optical counterpart is seen as the white spot centred in the cycle. In the lower right corner, the acquisition image of our HST COS observations is displayed. The 2$\stackrel{″}{.}$5 diameter aperture (shown as a red circle) encompasses the ULX, the He III bubble, and two neighbouring UV-bright stars. North is up and east is to the left.