We present a study of the irregular dwarf galaxy Holmberg Ⅱ based on ROSAT Position Sensitive Proportional Counter observations (total exposure time: 22 ks). Holmberg Ⅱ is a nearby (3.2 Mpc), well-studied dwarf irregular galaxy. It is famous for its interstellar medium (ISM), which is dominated by expanding structures such as H Ⅰ holes and shells. We search for X-ray emission from point sources as well as for diffuse emission, down to the detection limit of the ROSAT data. Using X-ray hardness ratio diagrams we differentiate between thermal plasma and power-law X-ray spectra, which helps to determine the nature of the individual sources. Correlating the X-ray data with complementary observations ranging from the far-ultraviolet to the radio regime, we increase the probability of correctly identifying sources belonging to Holmberg Ⅱ. We did not detect soft X-ray emission originating from hot gas within supergiant H Ⅰ shells above our luminosity sensitivity limit of L_(limit)(0.1-2.1 keV) ≥ 10~(37) ergs s~(-1). This finding can probably be attributed to blowout in the case of the largest holes and insufficient sensitivity (owing to strong photoelectric absorption) in the case of the smaller H Ⅰ holes. However, we find faint X-ray sources well beyond the stellar body but within the H Ⅰ distribution of Holmberg Ⅱ, which suggests the presence of X-ray binaries. This indicates that star formation has taken place across the entire gaseous disk of Holmberg Ⅱ in the past, some of which may have created the structures seen in the ISM at large galactocentric radii.
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