Observations made with the PSPC on board the Röntgensatellit (ROSAT) of the two nearby giant stars |β| CMa (B1 II-III) and |ε| CMa (B2 II) are presented. These stars are of special interest because the exceptional lack of any significant absorbing interstellar column towards them affords us an unhindered view of their soft X-ray emission. The PSPC data on both stars are examined for variability. Upper limits in the region of |±10| per cent can be set on the variation of the 0.1–0.4 keV and 0.5–2.0 keV fluxes from |β| CMa. More limited data on |ε| CMa set upper limits of around |± 20| per cent. The best model fit to the soft X-ray spectrum of both stars is a two-temperature optically thin plasma in which the lower |~2×10⁶- K| temperature component provides about three-quarters of the total emission measure. By attributing the soft X-ray emission to an outer zone of hot shocked gas enclosing the cool inner wind responsible for observed ultraviolet blueshifted absorption features, it is estimated that the mass-loss rates for |β| and |ε| CMa respectively are |(6±2)×10⁻⁹| and |(8±2)×10⁻⁹ \ M_ \ yr⁻¹|. This is significantly less than predicted by extrapolation of the |Lbol- M| relation to which O stars conform. These data are illustrative of the way in which the radiation-driving mechanism switches off at the low-luminosity limit. This and other issues raised by the observations are discussed.
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Drew et al. (1994) studied this question.