Nebular SED fitting of the Sunburst galaxy, referred to as HostM in the text is shown in the main left panel. It is moderately magnified and represents the emission from the entire galaxy. In the inset, the F555W-band image shows HostM with the elliptical aperture (red line) used to compute PSF-matched photometry and the knots 5.1, 5.2 and 5.3. The shaded green ellipses mark the photometric excess in the F160W band, due to nebular emission of [O III]λ4959 mainly due to the YMC 5.1. High S/N nebular SED-fit of YMC 5.1 is shown in the upper right panel, including both 5.1b and 5.1c to highlight the clear photometric jump in the F160W band, as confirmed from VLT/X-shooter observations (Appendix A). The inset shows the images of 5.1b,c and the adopted aperture in the F555W band (blue). Same SED-fit of the complex 5.3 h is shown in the bottom right panel as an example in which no evident nebular contribution is present in the F160W band. The inset shows the image of 5.3 h and the adopted aperture in the F555W band (blue ellipse).